ISO/IEC 21838-1:2021

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

General Information

• Status: Published
• Publication Date: 05-Aug-2021
• ICS: 01.040.35 - Information technology (Vocabularies)
• ICS: 35.060 - Languages used in information technology
• Technical Committee: ISO/IEC JTC 1/SC 32 - Data management and interchange
• Drafting Committee: ISO/IEC JTC 1/SC 32/WG 2 - MetaData
• Current Stage: 6060 - International Standard published
• Start Date: 06-Aug-2021
• Due Date: 08-Dec-2020
• Completion Date: 06-Aug-2021

Overview

ISO/IEC 21838-1:2021 - Information technology - Top-level ontologies (TLO) - Part 1: Requirements specifies the required characteristics for a domain-neutral top-level ontology (TLO) intended to act as a hub for interoperability across domain ontologies. The standard defines what a TLO must provide to support machine-readable exchange, retrieval, discovery, integration and analysis of data by computer systems. It focuses on coverage, documentation and representation of TLO content and on how a TLO relates to and underpins lower-level domain ontologies.

Key topics and technical requirements

• Domain neutrality and breadth of coverage: A TLO must be domain‑neutral and documented to demonstrate sufficiently broad conceptual coverage to serve as an upper-level hub for multiple domain ontologies.
• Machine‑readable axiomatizations: The TLO should be available in formal axiomatizations. The document normatively references two representations:
  • Common Logic (CL) for full first‑order expressivity (required to capture rich axioms).
  • OWL 2 (direct semantics) for a decidable, practically usable representation for reasoning and tooling.
• Textual artefact and supplementary documentation: A TLO must include human‑readable documentation explaining purpose, scope, intended use, and guidance for showing conformance of domain ontologies.
• Relations to domain ontologies: The standard specifies how TLO terms are intended to be used in definitions and axioms of lower‑level ontologies and how to demonstrate conformance.
• Conformance and evidence: Requirements for documentation and supporting materials that enable others to assess whether a TLO meets the standard’s requirements and whether a domain ontology conforms to it.
• Topics explicitly out of scope: The standard does not define ontology languages themselves, reasoning methods, translators between ontology notations, IRI permanence rules, ontology maintenance/versioning principles, or data tagging/annotation practices.

Applications and who uses it

• Primary uses: Designing multi‑level ontology suites, re‑engineering legacy ontologies to a coherent upper ontology, and enabling semantic interoperability across heterogeneous information systems.
• Who benefits:
  • Ontology engineers and semantic architects
  • Data architects and integration teams in healthcare, bioinformatics, manufacturing, government and defense
  • Consortia and large enterprises coordinating cross‑domain data exchange and analytics
• Practical outcomes: Improved semantic alignment across datasets and systems, clearer mapping strategies for domain ontologies, and more robust tooling and reasoning support through standardized representations.

Related standards

• ISO/IEC 24707 - Common Logic (CL)
• W3C OWL 2 recommendations (OWL 2 Web Ontology Language, Direct Semantics, and Syntax)
• Other parts of the ISO/IEC 21838 series (for further TLO guidance)

Keywords: ISO/IEC 21838-1:2021, top-level ontology, TLO, semantic interoperability, OWL 2, Common Logic, domain ontologies, axiomatization, ontology suite.

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 21838-1
First edition
2021-08
Information technology — Top-level
ontologies (TLO) —
Part 1:
Requirements
Reference number
©
ISO/IEC 2021
© 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
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CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO/IEC 2021 – All rights reserved

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

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
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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).
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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

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

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
© ISO/IEC 2021 – All rights reserved 1

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.
2 © ISO/IEC 2021 – All rights reserved

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)
© ISO/IEC 2021 – All rights reserved 3

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’.
4 © ISO/IEC 2021 – All rights reserved

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.
© ISO/IEC 2021 – All rights reserved 5

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

proven consistent using standard OWL reasoners. The axiomatization shall be interpretable in the CL
axiomatization described in 4.3.
[1]
In the OWL 2 axiomatization, terms and relational expressions are replaced by IRIs used in accordance
[2]
with the rules in the W3C Recommendation — OWL Web Ontology Language Guide .
4.2.2 Alternative OWL 2 Axiomatization
In some cases, in order to compensate for the restrictions on axiom closure in an OWL 2 ontology, a TLO
may be provided with two or more OWL 2 axiomatizations, neither of which is logically interpretable
in the other (W3C Recommendation — OWL 2 Web Ontology Language Structural Specification and
Functional-Style Syntax). Each such axiomatization shall however be logically interpretable in the CL
axiomatization and a specification shall be provided of how such OWL 2 axiomatizations relate to each
other and why each is needed.
NOTE 1 In the simplest case, the axiomatizations form a set linearly ordered in terms of theory strength, where
theory A is stronger than theory B when B is logically interpretable in A, but A is not logically interpretable in B.
Theory B is logically interpretable in theory A if, and only if, the language of B can be translated into the language
of A so that every theorem of B is derivable in A. An ontology developed in OWL 2 is always logically interpretable
in CL, but not vice versa.
NOTE 2 To define ‘axiom closure’, the import closure I(O) of an ontology O is first defined as the set
containing O and all the ontologies that O imports. The axiom closure of O is then the smallest set that contains
all the axioms in I(O) when the anonymous individuals from different ontologies in I(O) are treated as being
different (W3C Recommendation — OWL 2 Web Ontology Language Structural Specification and Functional-Style
Syntax).
4.3 Axiomatization in a CL-conforming language
The TLO shall be made available via an axiomatization in a language conforming to ISO/IEC 24707.
NOTE CL, a logical framework standardized for the purpose of facilitating exchange and transmission of
knowledge in computer-based systems, is the standard ontology development language defined in ISO/IEC 24707.
Many of the principles underlying a TLO – for example, regarding change, mereology, and temporal and spatial
location – cannot be adequately expressed using OWL but require the expressivity of first-order logic provided
by CL. CL is a family of formal languages with a common descriptive semantics. Since CL circumvents differences
in formal language syntax by focusing on a shared semantics, translations between distinct formal languages are
easier to automate.
EXAMPLES Languages conforming to CL specified in ISO/IEC 24707 are the Common Logic Interchange
Format (CLIF), the Conceptual Graph Interchange Format (CGIF), and the XML-based notation for Common Logic
(XCL). For details of how languages traditionally used in first-order logic (FOL) can also conform to ISO/IEC 24707,
see Reference [7].
The signature of the CL axiomatization shall be identical, modulo the conversion from strings into
symbols, to the set of natural language terms and relational expressions of the ontology as specified in
4.1. The axiomatization shall extend the OWL 2 DL axiomatization described in 4.3 in the sense that its
models shall also satisfy the CL translation of the OWL 2 axiomatization. The axiomatization shall be
proven consistent using standard automated theorem provers. The axiomatization shall be explicitly
modularized.
4.4 Supplementary documentation
4.4.1 Overview
Supplementary documentation shall be made publicly available:
— specifying how the ontology is used or is intended to be used;
— specifying how it is shown that the OWL axiomatization specified in 4.2 is logically derivable from
the CL axiomatization specified in 4.3;
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— demonstrating the breadth of coverage of the ontology by addressing the questions listed in 4.4.6;
— documenting policies for ontology management.
4.4.2 Documentation of the purpose of the TLO
The actual or envisaged purpose of the TLO shall be described in detail.
EXAMPLES Uses of a TLO as a framework:
a. for the development of domain ontologies or domain ontology suites (in Annex A);
b. for the development of knowledge bases drawing on data from multiple domains;
c. for the re-engineering of existing (‘legacy’) domain ontologies and similar information artefacts with the
purpose of advancing interoperability or to promote clarity of definitions;
d. for ontology support for systems integration initiatives (involving both humans and machines);
e. for high-level structuring of cross-domain lexical resources such as WordNet (for example for purposes of
disambiguation of polysemous expressions);
f. to regiment the terminological content of a scientific theory;
g. to support web-based cataloguing of large collections for example by museums or media organizations.
4.4.3 Documentation concerning demonstration of conformance of a domain ontology to the
TLO
Where, as in 4.4.2, EXAMPLES a-c, the TLO is used in conjunction with external domain ontology
resources in ways which require conformance of these resources to the TLO in accordance with
definition 3.23, documentation is required concerning (a) the mechanisms used to achieve such
conformance, (b) the methods used to demonstrate conformance. Recommended methods are outlined
in Annex D.
4.4.4 Documentation concerning consistency of the CL axiomatization
Documentation shall be provided which provides an interpretation that demonstrates that the CL
axiomatization is consistent, with instructions on how to verify satisfaction. This documentation
shall include a specification of the model used to prove consistency and an account of how the TLO is
modularized.
NOTE A set of CL axioms is consistent if the set of formulae derivable from the axioms using the standard
rules of inference does not contain a contradiction. Consistency can be proved either semantically or syntactically.
A semantic proof shows that the set of axioms has an interpretation (also called a model) in which all axioms are
satisfied. A syntactic proof can be either direct or indirect. The former proves directly that there is no formula
such that both it and its negation are derivable from the axioms. The latter proves consistency by using theorem
provers to show that a set of axioms is logically interpretable in a theory that has already been proved consistent.
This technique is used for theories that have no finite models.
4.4.5 Documentation concerning the relation between OWL and CL axiomatizations
Documentation shall be provided specifying how it is shown that the OWL 2 axiomatization specified in
4.2 is logically derivable from the CL axiomatization specified in 4.3. This documentation is required in
order to establish that the two axiomatizations can be accepted as axiomatizations of one and the same
ontology.
EXAMPLE It is shown that OWL 2 axiomatization A is logically interpretable in CL axiomatization B through
the following steps: 1. an automatic syntactic translator is used to convert A into CL(A) with CL-conforming
syntax; 2. translation definitions which bridge the signature of B to that of CL(A) are added to B, yielding the
result TR(B); 3. an automated theorem prover is used to show that the translation of each axiom of CL(A) is
entailed by TR(B).
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CL allows the development of ontologies stronger than those developed in OWL 2, in the sense that
an OWL 2 axiomatization is always logically interpretable in a CL axiomatization but not vice versa.
Given the differences in expressivity as between OWL 2 and CL, it may be necessary to use terms and
relational expressions in the OWL 2 axiomatization that are not present in the CL axiomatization and
vice versa. To show logical interpretability of the former in the latter it would then be necessary to add
the corresponding expressions and their definitions to the CL axiomatization. Such addition shall be
a conservative extension of the CL axiomatization, which means that any theorem formulable in the
extended ontology using only the old signature is already provable without the extension.
4.4.6 Documentation demonstrating breadth of coverage
4.4.6.1 Overview
The ontology documentation shall provide answers to the questions listed in subclauses 4.4.6.2 through
4.4.6.16. These answers shall document how the TLO would be used in managing data of the types
addressed in each subclause. (Annex C provides examples of such documentation.)
In some TLOs data about entities of given classes or types would be managed by using terms included
in the ontology representing those classes or types. Where a TLO does not include classes or types that
cover one or more of the areas identified, it shall be documented how it will address corresponding
data, for example, by specifying an additional ontology whose relation to the TLO is documented.
NOTE The rationale for requiring breadth of coverage in a TLO is as follows. When an ontology-based
approach is adopted, for example, by a large organization in order to promote interoperability of the data systems
within its constituent sub-organizations, the ontologies in question will be required to deal with an evolving
collection of different sorts of data. These will include:
— data that is spatially and temporally referenced;
— data about entities that change over time;
— data that result from assays along multiple qualitative and quantitative dimensions;
— data reflecting mereological and other relations between such entities, including relations between entities
and the material of which they are composed;
— data about data artefacts themselves (for example about designs, plans, requirements specifications).
If it is to have a high likelihood of being able to serve reliably as an over-arching framework for the
management of data in such circumstances – even when new sorts of data are being brought on
stream – then a TLO requires a maximal breadth of coverage in the set of terms it includes. Similarly,
a TLO should include relational expressions that enable representation of a broad range of relations
among entities in its chosen categories. Various candidate TLOs have made different – and sometimes
incompatible – choices concerning these categories and relations. To show conformity to this document,
these choices shall be documented in a way that will justify the claim that the ontology has a sufficiently
broad coverage of categories and associated relations to satisfy the requirements of a TLO as defined by
this document.
4.4.6.2 Space and time
How does the ontology deal with time, space and spacetime?
— Does the ontology recognize entities which persist in time?
— How does the ontology deal with entities which occur in time?
— Does the ontology recognize entities which are extended in both space and time?
— How does the ontology deal with spatial, temporal and spatiotemporal regions?
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4.4.6.3 Actuality and possibility
How does the ontology deal with what could happen or what could be the case, rather than what is the
case or has happened?
— How does the ontology deal with possibility?
— Does the ontology support both possible and actual entities?
— Does the ontology have a treatment of dispositions or tendencies?
— Does the ontology have a way of dealing with merely possible or potential entities as might be
described in unrealized plans or designs?
4.4.6.4 Classes and types
How does the ontology deal with issues of classification?
— Does classification reflect the existence of certain relations of similarity between certain entities,
or do classes or types exist as general entities in addition to particular instances?
— Are classes of classes allowed?
— Does the ontology distinguish between types and the classes of their instances?
— Are classes or types instantiated by the same particulars identical?
4.4.6.5 Time and change
How does the ontology deal with time and change?
— How does the ontology deal with the distinction between past, present and future entities?
— How does the ontology deal with identity and change of material objects over time?
— How does the ontology deal with location, and with change of location?
— Does the ontology allow for more than one material object to occupy exactly the same spatial
location at the same time?
— How does the ontology deal with changeable properties, such as being a student?
— Does the ontology recognize a distinction between classes or types that apply necessarily to a
particular for the whole of its existence, and classes or types that apply only temporarily?
EXAMPLES Mammal is an example of a class or type that applies to a particular for the whole of its existence.
An organism is an example of an entity that can undergo change over time, such as by losing hair, without
changing identity.
4.4.6.6 Parts, wholes, unity and boundaries
How does the ontology deal with relations of parthood?
— If one entity is part of a second entity, and this entity part of a third entity, does it follow that the
first entity is also part of the third entity?
— If one entity is part of but not identical to a second entity, must there be a third entity which makes
up the difference?
— How does the ontology deal with wholes formed through the summation of parts?
— How does the ontology deal with continuity where a material object has parts between which there
is no natural boundary?
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— How does it deal with the factor of unity, which obtains where the parts of a whole are joined
together in a way that distinguishes it from a sum?
EXAMPLES Unity is manifested by organisms or planets through the relation of direct or indirect physical
connectedness; unity is manifested by solar systems and galaxies through relations of gravity that are above
certain thresholds. Unity is manifested by a married couple through the relation of married to, and by a group of
siblings through the relation sibling of.
NOTE A whole manifesting the factor of unity can be defined as being such that all its parts are related to
each other, and only to each other, by a single distinguished relation.
4.4.6.7 Space and place
— How does the ontology deal with places and locations?
— How does the ontology deal with holes, conduits, cavities, a vacuum?
— How does the ontology deal with shape?
4.4.6.8 Scale and granularity
How does the ontology deal with scale, granularity and levels of reality?
— Does the
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