Information technology — Top-level ontologies (TLO) — Part 1: Requirements

This document specifies required characteristics of a domain-neutral top-level ontology (TLO) that can be used in tandem with domain ontologies at lower levels to support data exchange, retrieval, discovery, integration and analysis. If an ontology is to provide the overarching ontology content that will promote interoperability of domain ontologies and thereby support the design and use of purpose-built ontology suites, then it needs to satisfy certain requirements. This document specifies these requirements. It also supports a variety of other goals related to the achievement of semantic interoperability, for example, as concerns legacy ontologies developed using heterogeneous upper-level categories, where a coherently designed TLO can provide a target for coordinated re-engineering. This document specifies the characteristics an ontology needs to possess to support the goals of exchange, retrieval, discovery, integration and analysis of data by computer systems. The following are within the scope of this document — Specification of the requirements an ontology needs to satisfy if it is to serve as a top-level hub ontology. — Specification of the relations between a top-level ontology and domain ontologies. — Specification of the role played by the terms in a top-level ontology in the formulation of definitions and axioms in ontologies at lower levels. The following are outside the scope of this document: — Specification of ontology languages, including the languages OWL 2 and CL, used in ontology development with standard model-theoretic semantics. — Specification of methods for reasoning with ontologies. — Specification of translators between notations of ontologies developed in different ontology languages. — Specification of rules governing the use of IRIs as permanent identifiers for ontology terms. — Specification of the principles of ontology maintenance and versioning. — Specification of how ontologies can be used in the tagging or annotation of data.

Technologies de l'information — Ontologies de haut-niveau (TLO) — Partie 1: Exigences

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05-Aug-2021
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6060 - International Standard published
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INTERNATIONAL ISO/IEC
STANDARD 21838-1
First edition
2021-08
Information technology — Top-level
ontologies (TLO) —
Part 1:
Requirements
Reference number
ISO/IEC 21838-1:2021(E)
ISO/IEC 2021
---------------------- Page: 1 ----------------------
ISO/IEC 21838-1:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO/IEC 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC 21838-1:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 2

4 Requirements for a top-level ontology .......................................................................................................................................... 5

4.1 TLO as textual artefact ...................................................................................................................................................................... 5

4.1.1 Overview ................................................................................................................................................................................. 5

4.1.2 Relations between textual artefact and axiomatizations of the TLO .................................. 6

4.2 Axiomatization in the Web Ontology Language (OWL 2 with direct semantics) .......................... 6

4.2.1 General...................................................................................................................................................................................... 6

4.2.2 Alternative OWL 2 Axiomatization .................................................................................................................. 7

4.3 Axiomatization in a CL-conforming language .............................................................................................................. 7

4.4 Supplementary documentation ................................................................................................................................................ 7

4.4.1 Overview ................................................................................................................................................................................. 7

4.4.2 Documentation of the purpose of the TLO ................................................................................................ 8

4.4.3 Documentation concerning demonstration of conformance of a domain

ontology to the TLO ....................................................................................................................................................... 8

4.4.4 Documentation concerning consistency of the CL axiomatization ...................................... 8

4.4.5 Documentation concerning the relation between OWL and CL axiomatizations .... 8

4.4.6 Documentation demonstrating breadth of coverage........................................................................ 9

4.4.7 Domain neutrality ........................................................................................................................................................13

4.4.8 Ontology management ........................................................................................................................................... ..13

5 Conformity ...............................................................................................................................................................................................................13

5.1 Overview ...................................................................................................................................................................................................13

5.2 Ontology documentation .............................................................................................................................................................14

5.3 Supplementary documentation .............................................................................................................................................14

Annex A (informative) Examples of ontology suites ..........................................................................................................................15

Annex B (informative) The definition of ‘ontology’ ............................................................................................................................16

Annex C (informative) Examples of documentation demonstrating breadth of coverage .........................19

Annex D (informative) Conformance of a domain ontology to a TLO ..............................................................................21

Bibliography .............................................................................................................................................................................................................................23

© ISO/IEC 2021 – All rights reserved iii
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ISO/IEC 21838-1:2021(E)
Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical

Commission) form the specialized system for worldwide standardization. National bodies that are

members of ISO or IEC participate in the development of International Standards through technical

committees established by the respective organization to deal with particular fields of technical

activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other international

organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the

work.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for

the different types of document should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives or www .iec .ch/ members

_experts/ refdocs).

Attention is drawn to the possibility that some of the elements of this document may be the subject

of patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent

rights. Details of any patent rights identified during the development of the document will be in the

Introduction and/or on the ISO list of patent declarations received (see www .iso .org/ patents) or the IEC

list of patent declarations received (see patents.iec.ch).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/

iso/ foreword .html. In the IEC, see www .iec .ch/ understanding -standards.

This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,

Subcommittee SC 32, Data management and interchange.
A list of all parts in the ISO/IEC 21838 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html and www .iec .ch/ national

-committees.
iv © ISO/IEC 2021 – All rights reserved
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ISO/IEC 21838-1:2021(E)
Introduction

This document was developed in response to the demand from many quarters for ontology-based

solutions to the problem of semantic interoperability across networks of information systems. The

demand arises particularly from large organizations and consortia of organizations in areas such as

bioinformatics, healthcare, the manufacturing industry and military and government administration,

where independently created information systems need to exchange data in such a way that meaning is

preserved.

An ontology is on the one hand an artefact for human use, built out of terms and relations expressed

using natural language. On the other hand, it is an artefact for use by computers, which requires that

these terms and relations are captured in a formal language that is machine readable and has well-

defined (typically, model-theoretic) semantics. Multiple languages have been developed for the

purposes of ontology formalization, of which Common Logic (CL) and the Web Ontology Language

(OWL) – specifically OWL 2 with direct semantics – are normatively referenced in this document.

An ontology can help to achieve sharing of meaning because its terms are associated with formal

definitions specifying their meanings in a way that can be processed computationally. If an ontology can

be shared across participating organizations, then data can be exchanged in such a way that meaning is

preserved if the data can be associated with corresponding shared ontology terms.

CL and OWL 2 serve different ends. CL is a logical framework with the full expressivity of first-order

logic (FOL), the unifying framework for all semantic web applications. Formalization in a language with

the expressivity of FOL is required for the purposes of this document since weaker expressivity would

not allow the ontology to capture in a formal way the implications of axioms in areas such as mereology

and theories of location and change.

Formalization in a language like OWL 2 is needed, even though it is less expressive than CL, since it is

decidable and this means that it can be used effectively by computer systems for purposes of logical

reasoning and ontology quality assurance.

Where heterogeneous bodies of data need to be exchanged or manipulated, some have adopted

approaches that involve the creation of a suite of ontologies incorporating a distinction of levels, with a

single very general ontology at the top, governing one or more specific ontology modules at lower levels

(Annex A provides examples). This document addresses the need that arises for those communities

that have adopted such multi-level approaches. Specifically, its purpose is to specify what is required

of a top-level ontology if it is to serve the needs of those building or re-engineering ontologies or other

legacy systems at lower levels in a way that will support semantic interoperability among them.

To be fit for purpose, a top-level ontology needs to have appropriate content that is well documented and

be available in machine-readable forms providing support for computational reasoning. This document

specifies these requirements in terms of coverage, documentation and representation.

© ISO/IEC 2021 – All rights reserved v
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INTERNATIONAL STANDARD ISO/IEC 21838-1:2021(E)
Information technology — Top-level ontologies (TLO) —
Part 1:
Requirements
1 Scope

This document specifies required characteristics of a domain-neutral top-level ontology (TLO) that can

be used in tandem with domain ontologies at lower levels to support data exchange, retrieval, discovery,

integration and analysis.

If an ontology is to provide the overarching ontology content that will promote interoperability of

domain ontologies and thereby support the design and use of purpose-built ontology suites, then it

needs to satisfy certain requirements. This document specifies these requirements. It also supports a

variety of other goals related to the achievement of semantic interoperability, for example, as concerns

legacy ontologies developed using heterogeneous upper-level categories, where a coherently designed

TLO can provide a target for coordinated re-engineering.

This document specifies the characteristics an ontology needs to possess to support the goals of

exchange, retrieval, discovery, integration and analysis of data by computer systems.

The following are within the scope of this document

— Specification of the requirements an ontology needs to satisfy if it is to serve as a top-level hub

ontology.

— Specification of the relations between a top-level ontology and domain ontologies.

— Specification of the role played by the terms in a top-level ontology in the formulation of definitions

and axioms in ontologies at lower levels.
The following are outside the scope of this document:

— Specification of ontology languages, including the languages OWL 2 and CL, used in ontology

development with standard model-theoretic semantics.
— Specification of methods for reasoning with ontologies.

— Specification of translators between notations of ontologies developed in different ontology

languages.

— Specification of rules governing the use of IRIs as permanent identifiers for ontology terms.

— Specification of the principles of ontology maintenance and versioning.

— Specification of how ontologies can be used in the tagging or annotation of data.

2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO/IEC 24707, Information technology — Common Logic (CL) — A framework for a family of logic-based

languages
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ISO/IEC 21838-1:2021(E)

WORLD WIDE WEB CONSORTIUM W3C Recommendation — OWL 2 Web Ontology Language Document

Overview (Second Edition), https:// www .w3 .org/ TR/ 2012/ REC -owl2 -overview -20121211/

WORLD WIDE WEB CONSORTIUM W3C Recommendation — OWL 2 Web Ontology Language Direct

Semantics, https:// www .w3 .org/ TR/ owl2 -direct -semantics/

WORLD WIDE WEB CONSORTIUM W3C Recommendation — OWL 2 Web Ontology Language Structural

Specification and Functional-Style Syntax (Second Edition), http:// www .w3 .org/ TR/ 2012/ REC -owl2

-syntax -20121211/
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/

NOTE The following terms and definitions are not intended as a substitute for existing technical vocabularies

used in ontology development and maintenance, for example, as defined by the W3C. To reduce the possibility of

confusion, expressions used in describing a W3C recommended usage are capitalized.

3.1
entity
object
item that is perceivable or conceivable

Note 1 to entry: The terms ‘entity’ and ‘object’ are catch-all terms analogous to ‘something’. In terminology circles

‘object’ is commonly used in this way. In ontology circles, ‘entity’ and ‘thing’ are commonly used. See B.3.3.

[SOURCE: ISO 1087-1:2000]
3.2
class
general entity (3.1)

Note 1 to entry: In some ontology communities, all general entities are referred to as classes. In other ontology

communities, a distinction is drawn between classes as the extensions of general entities (for example, as sets

of instances) and the general entities themselves, sometimes referred to as ‘types’, ‘kinds’, or ‘universals’. The

expression ‘class or type’ is used in this document in order to remain neutral regarding these different usages.

3.3
particular
individual entity (3.1)

Note 1 to entry: In contrast to classes or types, particulars are not exemplified or instantiated by further entities.

3.4
relation
way in which entities (3.1) are related

Note 1 to entry: Relations can hold between particulars (this leg is part of this lion); or between classes or

types (mammal is a subclass of organism); or between particulars and classes or types (this lion is an instance of

mammal). On some views, identity is treated as a relation connecting one entity to itself.

Note 2 to entry: On the difference between ‘relation’ and ‘relational expression’ see 3.6, Note 1 to entry.

Note 3 to entry: ‘Relation’ is a primitive term. See 4.1.1, NOTE 1.
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ISO/IEC 21838-1:2021(E)
3.5
expression

word or group of words or corresponding symbols that can be used in making an assertion

Note 1 to entry: Expressions are divided into natural language expressions and expressions in a formal language.

3.6
relational expression
expression (3.5) used to assert that a relation (3.4) obtains

EXAMPLE ‘is a’ (also known as ‘subtype’ or ‘subclass’), ‘part of’, ‘member of’, ‘instantiates’ ‘later than’,

‘brother of’, ‘temperature of’.

Note 1 to entry: The term ‘relational expression’ is introduced in order to remove any confusion that can arise if

a person uses ‘relation’ to refer to the real-world link or bond between entities (as in 3.4), while another person

uses ‘relation’ to refer to the linguistic representation of this real-world link or bond.

Note 2 to entry: In OWL 2, relational expressions are referred to as Properties. ‘Expression’ is used to connote

logical composition: a Class Name in OWL 2 is logically simple, a Class Expression is logically complex. In FOL,

‘n-ary predicate’ is often used as a synonym of ‘relational expression’.
3.7
term
expression (3.5) that refers to some class (3.2) or to some particular (3.3)

Note 1 to entry: An ontology will typically contain a unique ‘preferred term’ for the entities within its coverage

domain. Preferred terms may then be supplemented with other terms recognized by the ontology as synonyms

of the preferred terms.
3.8
definition
concise statement of the meaning of an expression (3.5)

Note 1 to entry: For the purposes of this document, definitions can be of two sorts: (1) those formulated using

a natural language such as English, supplemented where necessary by technical terms or codes used in some

specialist domain; (2) those formulated using a computer-interpretable language such as OWL 2 or CL.

3.9
axiom
statement that is taken to be true, to serve as a premise for further reasoning

Note 1 to entry: Axioms may be formulated as natural language sentences or as formulae in a formal language. In

the OWL community, ‘Axiom’ is used to refer to statements that say what is true in the domain that are ‘basic’ in

the sense that they are not inferred from other statements.
3.10
formal language
language that is machine readable and has well-defined semantics

Note 1 to entry: Well-defined semantics will typically be model-theoretic semantics.

3.11
formal theory

collection of definitions (3.8) and axioms (3.9) expressed in a formal language (3.10)

Note 1 to entry: In some formal theories, definitions are expressed by means of axioms.

3.12
axiomatization

result of expressing a body of knowledge or information as a formal theory (3.11)

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ISO/IEC 21838-1:2021(E)
3.13
logical interpretability

ability to derive each and every axiom (3.9) of one formal theory (3.11) from another

Note 1 to entry: One formal theory is logically interpretable in a second formal theory if the language of the first

can be translated into the language of the second so that the translation of every axiom in the first is derivable

from the second.
3.14
ontology

collection of terms (3.7), relational expressions (3.6) and associated natural-language definitions (3.8)

together with one or more formal theories (3.11) designed to capture the intended interpretations of

these definitions

Note 1 to entry: Background materials on the sources, rationale and interpretation of this definition are provided

in Annex B.
3.15
signature
set of non-logical symbols of a formal language (3.10) or formal theory (3.11)

Note 1 to entry: The signature of an ontology consists of a set of terms (3.7) and relational expressions (3.6).

3.16
knowledge base

combination of an ontology (3.14) with a collection of data which terms (3.7) in the ontology have been

used to describe, classify or connect.
3.17
domain
collection of entities (3.1) of interest to a certain community or discipline

EXAMPLE The domain of agriculture, the domain of cell biology, the domain of aircraft maintenance, the

domain of philately.

Note 1 to entry: ‘Entities of interest’ can include both particulars and classes or types. The definition is to be

interpreted as meaning that a domain is a collection of entities that is narrow in scope. Thus, there is no universal

[21]

domain, to which everything would belong. Compare with ISO/IEC 2382 , which defines ‘domain model’ in the

context of artificial intelligence as: model of a specific field of knowledge or expertise.

3.18
domain ontology

ontology (3.14) whose terms (3.7) represent classes (3.2) or types and, optionally, certain particulars

(3.3) (called ‘distinguished individuals’) in some domain (3.17)
3.19
category

general class (3.2) or type that is shared across many different domains (3.17) and is represented by a

domain-neutral term (3.7)
EXAMPLE Process, attribute, event, region, information entity.
3.20
top-level ontology
TLO

ontology (3.14) that is created to represent the categories (3.19) that are shared across a maximally

broad range of domains (3.17)
[5]

Note 1 to entry: Top-level ontologies are ‘reference ontologies’ in the sense of ISO/IEC 19763-3 , A top-level

ontology is sometimes referred to as a ‘formal ontology’, ‘foundational ontology’, ‘upper level ontology’, or

‘domain-neutral ontology’.
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ISO/IEC 21838-1:2021(E)
3.21
ontology suite

collection of ontologies (3.14) developed in such a way as to be mutually consistent and non-redundant

Note 1 to entry: See Annex A.
3.22
ontology reuse

importing an ontology (3.14), or part of an ontology, into a second ontology in such a way as to preserve

the meaning of the imported content

EXAMPLE Terms from a tool ontology are reused in a power tool ontology; the latter is a specialization of

the former.

Note 1 to entry: Terms from the existing ontology will typically be reused in the new ontology and appear

together with the newly created terms.
3.23
ontology conformance

relation (3.4) between two ontologies (3.14) when one consistently extends the other

EXAMPLE A power tool ontology stands in the relation of ontological conformance to a tool ontology if the

former is a consistent ontology that results from adding new content (terms, definitions, axioms) to the latter.

Note 1 to entry: ‘Extension’ means semantically that any element in a model of the extending ontology which

satisfies the conditions for being an instance of a class in the starting ontology must be an instance of that class

in the extending ontology.

Note 2 to entry: This is a narrowly defined usage of ‘conformance’ that is intended to be used only in contexts in

which relations between ontologies are at issue. Where conformance in the sense of fulfilment of a requirement or

satisfaction of a criterion is intended in this document, the term ‘conformity’ is used.

4 Requirements for a top-level ontology
4.1 TLO as textual artefact
4.1.1 Overview

A TLO shall include a textual artefact represented by a natural language document providing: (1) a list

of domain-neutral terms and relational expressions, incorporating identification of primitive terms,

and (2) definitions of the meanings of the terms and relational expressions listed. Natural-language

definitions may incorporate semi-formal elements if these are needed for readability.

NOTE 1 In the case of primitive terms, definitions can take the form of elucidations of meaning supplemented

by examples of use.
EXAMPLE An example of a definition with semi-formal elements is:

transitivity =def. relation R is transitive if whenever a stands in R to b and b stands in R to c it follows that a

stands in R to c.

Given the nature of a TLO, a portion of its terms and relational expressions will be so basic in their

meaning that there will be no logically simpler, and thus more easily intelligible, expressions on the

basis of which they can be defined in a non-circular way. Ontology terms and relational expressions for

which this is the case are called ‘primitives’, and they have definitions in the sense of 3.8, but these are

circular or are mere paraphrases.
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ISO/IEC 21838-1:2021(E)

A TLO shall specify which of its terms and relational expressions are primitive in this sense. For all

other terms and relational expressions in the TLO, definitions shall be provided which satisfy the

conditions that:
a) they are non-circular;
b) they form a consistent set;
c) they are concise.

NOTE 2 Concise signifies that the definition contains no redundant elements (for example, lists of examples,

explanations of usage, and so on).

These requirements apply both to the natural language definitions and also to the definitions provided

in the OWL 2 and CL axiomatizations referenced in 4.2 and 4.3.

Non-circularity excludes not only immediate circularity (where the defined term or a term with

equivalent meaning is used in the definition) but also mediated circularity (for example, where a term is

used in the definition of a second term, which is itself used in the definition of the first term). To ensure

non-circularity it is recommended that definitions are formulated as statements of singly necessary

and jointly sufficient conditions for the correct application of the defined term.

EXAMPLE Triangle = def. closed figure that lies in a plane and consists of exactly three straight lines.

Consistency of the collection of natural language definitions is shown through the development of an

axiomatization that is proven consistent, as described in 4.2 and 4.3.

NOTE 3 Consistency, non-circularity and conciseness of definitions are features that distinguish ontologies

from traditional dictionaries and other lexical resources.
4.1.2 Relations between textual artefact and axiomatizations of the TLO

The terms and relational expressions in the textual artefact shall be converted into symbols in the

axiomatizations. These symbols together form the signature of the resultant logical theory. They may

incorporate textual strings.
EXAMPLE The text string ‘is a’ is converted into the symbol ‘is_a’.

Terms and relational expressions in the textual artefact should have counterparts in the OWL 2

axiomatization wherever this is feasible, given the expressivity of OWL.

Each definition in the textual artefact whose content is expressible in OWL 2 shall correspond in the

OWL 2 axiomatization to a group of one or more axioms with a corresponding logical content.

All terms in the textual artefact shall correspond to terms in the CL axiomatization.

All definitions of non-primitive terms in the textual artefact shall correspond to axioms in the CL

formalization.
4.2 Axiomatization in the Web Ontology Language (OWL 2 with direct semantics)
4.2.1 General

The TLO shall be made available via at least one machine-readable axiomatization in OWL 2 with

the direct semantics or in some description logic that is designated by W3C as a successor of OWL 2.

The signature of the OWL axiomatization shall be identical, modulo the conversion from strings into

symbols and modulo the conversion of ternary into binary relational expressions, to the set of natural

language terms and relational expressions of the TLO as specified under 4.1. The axioms should

represent the content of the natural language definitions described in 4.1 to the extent that this is

possible given the expressivity of OWL 2. The axiomatization shall satisfy the conformity criteria in

W3C Recommendation — OWL 2 Web Ontology Language Direct Semantics. The axiomatization shall be

6 © ISO/IEC 2021 – All rights reserved
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ISO/IEC 21838-1:2021(E)
...

INTERNATIONAL ISO/IEC
STANDARD 21838-1
First edition
Information technology — Top-level
ontologies (TLO) —
Part 1:
Requirements
Technologies de l'information — Haut-niveau ontologies
PROOF/ÉPREUVE
Reference number
ISO/IEC 21838-1:2020(E)
ISO/IEC 2020
---------------------- Page: 1 ----------------------
ISO/IEC 21838-1:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO/IEC 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC 21838-1:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 2

4 Requirements for a top-level ontology .......................................................................................................................................... 5

4.1 TLO as textual artefact ...................................................................................................................................................................... 5

4.1.1 Overview ................................................................................................................................................................................. 5

4.1.2 Relations between textual artefact and axiomatizations of the TLO .................................. 6

4.2 Axiomatization in the Web Ontology Language (OWL 2 with direct semantics) .......................... 6

4.2.1 General...................................................................................................................................................................................... 6

4.2.2 Alternative OWL 2 Axiomatization .................................................................................................................. 7

4.3 Axiomatization in a CL-conforming language .............................................................................................................. 7

4.4 Supplementary documentation ................................................................................................................................................ 7

4.4.1 Overview ................................................................................................................................................................................. 7

4.4.2 Documentation of the purpose of the TLO ................................................................................................ 8

4.4.3 Documentation concerning demonstration of conformance of a domain

ontology to the TLO ....................................................................................................................................................... 8

4.4.4 Documentation concerning consistency of the CL axiomatization ...................................... 8

4.4.5 Documentation concerning the relation between OWL and CL axiomatizations .... 8

4.4.6 Documentation demonstrating breadth of coverage........................................................................ 9

4.4.7 Domain neutrality ........................................................................................................................................................13

4.4.8 Ontology management ........................................................................................................................................... ..13

5 Conformity ...............................................................................................................................................................................................................13

5.1 Overview ...................................................................................................................................................................................................13

5.2 Ontology documentation .............................................................................................................................................................14

5.3 Supplementary documentation .............................................................................................................................................14

Annex A (informative) Examples of ontology suites ..........................................................................................................................15

Annex B (informative) The definition of ‘ontology’ ............................................................................................................................16

Annex C (informative) Examples of documentation demonstrating breadth of coverage .........................19

Annex D (informative) Conformance of a domain ontology to a TLO ..............................................................................21

Bibliography .............................................................................................................................................................................................................................23

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ISO/IEC 21838-1:2020(E)
Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical

Commission) form the specialized system for worldwide standardization. National bodies that

are members of ISO or IEC participate in the development of International Standards through

technical committees established by the respective organization to deal with particular fields of

technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other

international organizations, governmental and non-governmental, in liaison with ISO and IEC, also

take part in the work.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for

the different types of document should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject

of patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent

rights. Details of any patent rights identified during the development of the document will be in the

Introduction and/or on the ISO list of patent declarations received (see www .iso .org/ patents) or the IEC

list of patent declarations received (see http:// patents .iec .ch).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/

iso/ foreword .html.

This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,

Subcommittee SC 32, Data management and interchange.
A list of all parts in the ISO/IEC 21838 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO/IEC 21838-1:2020(E)
Introduction

This document was developed in response to the demand from many quarters for ontology-based

solutions to the problem of semantic interoperability across networks of information systems. The

demand arises particularly from large organizations and consortia of organizations in areas such as

bioinformatics, healthcare, the manufacturing industry and military and government administration,

where independently created information systems need to exchange data in such a way that meaning is

preserved.

An ontology is on the one hand an artefact for human use, built out of terms and relations expressed

using natural language. On the other hand, it is an artefact for use by computers, which requires that

these terms and relations are captured in a formal language that is machine readable and has well-

defined (typically, model-theoretic) semantics. Multiple languages have been developed for the

purposes of ontology formalization, of which Common Logic (CL) and the Web Ontology Language

(OWL) – specifically OWL 2 with direct semantics – are normatively referenced in this document.

An ontology can help to achieve sharing of meaning because its terms are associated with formal

definitions specifying their meanings in a way that can be processed computationally. If an ontology can

be shared across participating organizations, then data can be exchanged in such a way that meaning is

preserved if the data can be associated with corresponding shared ontology terms.

CL and OWL 2 serve different ends. CL is a logical framework with the full expressivity of first-order

logic (FOL), the unifying framework for all semantic web applications. Formalization in a language with

the expressivity of FOL is required for the purposes of this document since weaker expressivity would

not allow the ontology to capture in a formal way the implications of axioms in areas such as mereology

and theories of location and change.

Formalization in a language like OWL 2 is needed, even though it is less expressive than CL, since it is

decidable and this means that it can be used effectively by computer systems for purposes of logical

reasoning and ontology quality assurance.

Where heterogeneous bodies of data need to be exchanged or manipulated, some have adopted

approaches that involve the creation of a suite of ontologies incorporating a distinction of levels, with a

single very general ontology at the top, governing one or more specific ontology modules at lower levels

(Annex A provides examples). This document addresses the need that arises for those communities

that have adopted such multi-level approaches. Specifically, its purpose is to specify what is required

of a top-level ontology if it is to serve the needs of those building or re-engineering ontologies or other

legacy systems at lower levels in a way that will support semantic interoperability among them.

To be fit for purpose, a top-level ontology needs to have appropriate content that is well documented and

be available in machine-readable forms providing support for computational reasoning. This document

specifies these requirements in terms of coverage, documentation and representation.

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INTERNATIONAL STANDARD ISO/IEC 21838-1:2020(E)
Information technology — Top-level ontologies (TLO) —
Part 1:
Requirements
1 Scope

This document specifies required characteristics of a domain-neutral top-level ontology (TLO) that can

be used in tandem with domain ontologies at lower levels to support data exchange, retrieval, discovery,

integration and analysis.

If an ontology is to provide the overarching ontology content that will promote interoperability of

domain ontologies and thereby support the design and use of purpose-built ontology suites, then it

needs to satisfy certain requirements. This document specifies these requirements. It also supports a

variety of other goals related to the achievement of semantic interoperability, for example, as concerns

legacy ontologies developed using heterogeneous upper-level categories, where a coherently designed

TLO can provide a target for coordinated re-engineering.

This document specifies the characteristics an ontology needs to possess to support the goals of

exchange, retrieval, discovery, integration and analysis of data by computer systems.

The following are within the scope of this document:

— Specification of the requirements an ontology needs to satisfy if it is to serve as a top-level hub

ontology.

— Specification of the relations between a top-level ontology and domain ontologies.

— Specification of the role played by the terms in a top-level ontology in the formulation of definitions

and axioms in ontologies at lower levels.
The following are outside the scope of this document:

— Specification of ontology languages, including the languages OWL 2 and CL, used in ontology

development with standard model-theoretic semantics.
— Specification of methods for reasoning with ontologies.

— Specification of translators between notations of ontologies developed in different ontology

languages.

— Specification of rules governing the use of IRIs as permanent identifiers for ontology terms.

— Specification of the principles of ontology maintenance and versioning.

— Specification of how ontologies can be used in the tagging or annotation of data.

2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO/IEC 24707, Information technology — Common Logic (CL) — A framework for a family of logic-based

languages
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ISO/IEC 21838-1:2020(E)

WORLD WIDE WEB CONSORTIUM W3C Recommendation — OWL 2 Web Ontology Language Document

Overview (Second Edition), https:// www .w3 .org/ TR/ 2012/ REC -owl2 -overview -20121211/

WORLD WIDE WEB CONSORTIUM W3C Recommendation — OWL 2 Web Ontology Language Direct

Semantics, https:// www .w3 .org/ TR/ owl2 -direct -semantics/

WORLD WIDE WEB CONSORTIUM W3C Recommendation — OWL 2 Web Ontology Language Structural

Specification and Functional-Style Syntax (Second Edition), http:// www .w3 .org/ TR/ 2012/ REC -owl2

-syntax -20121211/
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/

NOTE The following terms and definitions are not intended as a substitute for existing technical vocabularies

used in ontology development and maintenance, for example, as defined by the W3C. To reduce the possibility of

confusion, expressions used in describing a W3C recommended usage are capitalized.

3.1
entity
object
item that is perceivable or conceivable

Note 1 to entry: The terms ‘entity’ and ‘object’ are catch-all terms analogous to ‘something’. In terminology circles

‘object’ is commonly used in this way. In ontology circles, ‘entity’ and ‘thing’ are commonly used. See B.3.3.

[SOURCE: ISO 1087-1:2000]
3.2
class
general entity (3.1)

Note 1 to entry: In some ontology communities, all general entities are referred to as classes. In other ontology

communities, a distinction is drawn between classes as the extensions of general entities (for example, as sets

of instances) and the general entities themselves, sometimes referred to as ‘types’, ‘kinds’, or ‘universals’. The

expression ‘class or type’ is used in this document in order to remain neutral regarding these different usages.

3.3
particular
individual entity (3.1)

Note 1 to entry: In contrast to classes or types, particulars are not exemplified or instantiated by further entities.

3.4
relation
way in which entities (3.1) are related

Note 1 to entry: Relations can hold between particulars (this leg is part of this lion); or between classes or

types (mammal is a subclass of organism); or between particulars and classes or types (this lion is an instance of

mammal). On some views, identity is treated as a relation connecting one entity to itself.

Note 2 to entry: On the difference between ‘relation’ and ‘relational expression’ see 3.6, Note 1 to entry.

Note 3 to entry: ‘Relation’ is a primitive term. See 4.1.1, NOTE 1.
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3.5
expression

word or group of words or corresponding symbols that can be used in making an assertion

Note 1 to entry: Expressions are divided into natural language expressions and expressions in a formal language.

3.6
relational expression
expression (3.5) used to assert that a relation (3.4) obtains

EXAMPLE ‘is a’ (also known as ‘subtype’ or ‘subclass’), ‘part of’, ‘member of’, ‘instantiates’ ‘later than’,

‘brother of’, ‘temperature of’.

Note 1 to entry: The term ‘relational expression’ is introduced in order to remove any confusion that can arise if

a person uses ‘relation’ to refer to the real-world link or bond between entities (as in 3.4), while another person

uses ‘relation’ to refer to the linguistic representation of this real-world link or bond.

Note 2 to entry: In OWL 2, relational expressions are referred to as Properties. ‘Expression’ is used to connote

logical composition: a Class Name in OWL 2 is logically simple, a Class Expression is logically complex. In FOL,

‘n-ary predicate’ is often used as a synonym of ‘relational expression’.
3.7
term
expression (3.5) that refers to some class (3.2) or to some particular (3.3)

Note 1 to entry: An ontology will typically contain a unique ‘preferred term’ for the entities within its coverage

domain. Preferred terms may then be supplemented with other terms recognized by the ontology as synonyms

of the preferred terms.
3.8
definition
concise statement of the meaning of an expression (3.5)

Note 1 to entry: For the purposes of this document, definitions can be of two sorts: (1) those formulated using

a natural language such as English, supplemented where necessary by technical terms or codes used in some

specialist domain; (2) those formulated using a computer-interpretable language such as OWL 2 or CL.

3.9
axiom
statement that is taken to be true, to serve as a premise for further reasoning

Note 1 to entry: Axioms may be formulated as natural language sentences or as formulae in a formal language. In

the OWL community, ‘Axiom’ is used to refer to statements that say what is true in the domain that are ‘basic’ in

the sense that they are not inferred from other statements.
3.10
formal language
language that is machine readable and has well-defined semantics

Note 1 to entry: Well-defined semantics will typically be model-theoretic semantics.

3.11
formal theory

collection of definitions (3.8) and axioms (3.9) expressed in a formal language (3.10)

Note 1 to entry: In some formal theories, definitions are expressed by means of axioms.

3.12
axiomatization

result of expressing a body of knowledge or information as a formal theory (3.11)

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3.13
logical interpretability

ability to derive each and every axiom (3.9) of one formal theory (3.11) from another

Note 1 to entry: One formal theory is logically interpretable in a second formal theory if the language of the first

can be translated into the language of the second so that the translation of every axiom in the first is derivable

from the second.
3.14
ontology

collection of terms (3.7), relational expressions (3.6) and associated natural-language definitions (3.8)

together with one or more formal theories (3.11) designed to capture the intended interpretations of

these definitions

Note 1 to entry: Background materials on the sources, rationale and interpretation of this definition are provided

in Annex B.
3.15
signature
set of non-logical symbols of a formal language (3.10) or formal theory (3.11)

Note 1 to entry: The signature of an ontology consists of a set of terms (3.7) and relational expressions (3.6).

3.16
knowledge base

combination of an ontology (3.14) with a collection of data which terms (3.7) in the ontology have been

used to describe, classify or connect.
3.17
domain
collection of entities (3.1) of interest to a certain community or discipline

EXAMPLE The domain of agriculture, the domain of cell biology, the domain of aircraft maintenance, the

domain of philately.

Note 1 to entry: ‘Entities of interest’ can include both particulars and classes or types. The definition is to be

interpreted as meaning that a domain is a collection of entities that is narrow in scope. Thus, there is no universal

[21]

domain, to which everything would belong. Compare with ISO/IEC 2382 , which defines ‘domain model’ in the

context of artificial intelligence as: model of a specific field of knowledge or expertise.

3.18
domain ontology

ontology (3.14) whose terms (3.7) represent classes (3.2) or types and, optionally, certain particulars

(3.3) (called ‘distinguished individuals’) in some domain (3.17)
3.19
category

general class (3.2) or type that is shared across many different domains (3.17) and is represented by a

domain-neutral term (3.7)
EXAMPLE Process, attribute, event, region, information entity.
3.20
top-level ontology
TLO

ontology (3.14) that is created to represent the categories (3.19) that are shared across a maximally

broad range of domains (3.17)
[5]

Note 1 to entry: Top-level ontologies are ‘reference ontologies’ in the sense of ISO/IEC 19763-3 , A top-level

ontology is sometimes referred to as a ‘formal ontology’, ‘foundational ontology’, ‘upper level ontology’, or

‘domain-neutral ontology’.
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3.21
ontology suite

collection of ontologies (3.14) developed in such a way as to be mutually consistent and non-redundant

Note 1 to entry: See Annex A.
3.22
ontology reuse

importing an ontology (3.14), or part of an ontology, into a second ontology in such a way as to preserve

the meaning of the imported content

EXAMPLE Terms from a tool ontology are reused in a power tool ontology; the latter is a specialization of

the former.

Note 1 to entry: Terms from the existing ontology will typically be reused in the new ontology and appear

together with the newly created terms.
3.23
ontology conformance

relation (3.4) between two ontologies (3.14) when one consistently extends the other

EXAMPLE A power tool ontology stands in the relation of ontological conformance to a tool ontology if the

former is a consistent ontology that results from adding new content (terms, definitions, axioms) to the latter.

Note 1 to entry: ‘Extension’ means semantically that any element in a model of the extending ontology which

satisfies the conditions for being an instance of a class in the starting ontology must be an instance of that class

in the extending ontology.

Note 2 to entry: This is a narrowly defined usage of ‘conformance’ that is intended to be used only in contexts in

which relations between ontologies are at issue. Where conformance in the sense of fulfilment of a requirement or

satisfaction of a criterion is intended in this document, the term ‘conformity’ is used.

4 Requirements for a top-level ontology
4.1 TLO as textual artefact
4.1.1 Overview

A TLO shall include a textual artefact represented by a natural language document providing: (1) a list

of domain-neutral terms and relational expressions, incorporating identification of primitive terms,

and (2) definitions of the meanings of the terms and relational expressions listed. Natural-language

definitions may incorporate semi-formal elements if these are needed for readability.

NOTE 1 In the case of primitive terms, definitions can take the form of elucidations of meaning supplemented

by examples of use.
EXAMPLE An example of a definition with semi-formal elements is:

transitivity =def. relation R is transitive if whenever a stands in R to b and b stands in R to c it follows that a

stands in R to c.

Given the nature of a TLO, a portion of its terms and relational expressions will be so basic in their

meaning that there will be no logically simpler, and thus more easily intelligible, expressions on the

basis of which they can be defined in a non-circular way. Ontology terms and relational expressions for

which this is the case are called ‘primitives’, and they have definitions in the sense of 3.8, but these are

circular or they involve mere paraphrase in ways which involve using expressions that are themselves

undefined.
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A TLO shall specify which of its terms and relational expressions are primitive in this sense. For all

other terms and relational expressions in the TLO, definitions shall be provided which satisfy the

conditions that:
a) they are non-circular;
b) they form a consistent set;
c) they are concise.

NOTE 2 Concise signifies that the definition contains no redundant elements (for example, lists of examples,

explanations of usage, and so on).

These requirements apply both to the natural language definitions and also to the definitions provided

in the OWL 2 and CL axiomatizations referenced in 4.2 and 4.3.

Non-circularity excludes not only immediate circularity (where the defined term or a term with

equivalent meaning is used in the definition) but also mediated circularity (for example, where a term is

used in the definition of a second term, which is itself used in the definition of the first term). To ensure

non-circularity it is recommended that definitions are formulated as statements of singly necessary

and jointly sufficient conditions for the correct application of the defined term.

EXAMPLE Triangle = def. closed figure that lies in a plane and consists of exactly three straight lines.

Consistency of the collection of natural language definitions is shown through the development of an

axiomatization that is proven consistent, as described in 4.2 and 4.3.

NOTE 3 Consistency, non-circularity and conciseness of definitions are features that distinguish ontologies

from traditional dictionaries and other lexical resources.
4.1.2 Relations between textual artefact and axiomatizations of the TLO

The terms and relational expressions in the textual artefact shall be converted into symbols in the

axiomatizations. These symbols together form the signature of the resultant logical theory. They may

incorporate textual strings.
EXAMPLE The text string ‘is a’ is converted into the symbol ‘is_a’.

Terms and relational expressions in the textual artefact should have counterparts in the OWL 2

axiomatization wherever this is feasible, given the expressivity of OWL.

Each definition in the textual artefact whose content is expressible in OWL 2 shall correspond in the

OWL 2 axiomatization to a group of one or more axioms with a corresponding logical content.

All terms in the textual artefact shall correspond to terms in the CL axiomatization.

All definitions of non-primitive terms in the textual artefact shall correspond to axioms in the CL

formalization.
4.2 Axiomatization in the Web Ontology Language (OWL 2 with direct semantics)
4.2.1 General

The TLO shall be made available via at least one machine-readable axiomatization in OWL 2 with

the direct semantics or in some description logic that is designated by W3C as a successor of OWL 2.

The signature of the OWL axiomatization shall be identical, modulo the conversion from strings into

symbols and modulo the conversion of ternary into binary relational expressions, to the set of natural

language terms and relational expressions of the TLO as specified under 4.1. The axioms should

represent the content of the natural language definitions described in 4.1 to the extent that this is

possible given the expressivity of OWL 2. The axiomatization shall satisfy the conformity criteria in

W3C Recommendation — OWL 2 Web Ontology Language Direct Semantics. The a
...

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