ISO/IEC 19503:2005

Information technology — XML Metadata Interchange (XMI)

The main purpose of ISO/IEC 19503:2005 (XMI) is to enable easy interchange of metadata between application development lifecycle tools (such as modeling tools based on the Unified Modeling Language (UML), ISO/IEC 19501, and metadata repositories/frameworks based on the Meta Object Facility (MOF), ISO/IEC 19502) in distributed heterogeneous environments. ISO/IEC 19503:2005 integrates three key industry standards: XML, eXtensible Markup Language, a W3C standard; UML, Unified Modeling Language, an OMG modeling specification which is now ISO/IEC 19501; MOF, Meta Object Facility (ISO/IEC 19502). ISO/IEC 19503:2005 provides specifications for a set of XML Schema Definitions (XSD) production rules for transforming MOF-based metamodels into XML Schemas; a set of XML Document production rules for encoding and decoding MOF-based metadata; design principles for XMI-based Schemas and XML documents; a set of production rules for importing XML DTDs to a MOF-based metamodel. ISO/IEC 19503:2005 enhances metadata management and metadata interoperability in distributed object environments in general and in distributed development environments in particular. While it addresses stream-based metadata interoperability in the object analysis and design domain, ISO/IEC 19503:2005 (in part because it is MOF based) is equally applicable to metadata in many other domains.

Technologies de l'information — Échange de métadonnées XML (XMI)

General Information

• Status: Published
• Publication Date: 07-Nov-2005
• ICS: 35.040 - Information coding
• ICS: 35.040.50 - Automatic identification and data capture techniques
• Technical Committee: ISO/IEC JTC 1/SC 32 - Data management and interchange
• Drafting Committee: ISO/IEC JTC 1/SC 32 - Data management and interchange
• Current Stage: 9093 - International Standard confirmed
• Start Date: 03-Dec-2021
• Completion Date: 14-Feb-2026

Overview

ISO/IEC 19503:2005 - Information technology - XML Metadata Interchange (XMI) defines a standard mechanism to interchange metadata between application development lifecycle tools and metadata repositories in distributed, heterogeneous environments. XMI integrates three industry technologies - XML (W3C), UML (ISO/IEC 19501) and MOF (ISO/IEC 19502) - to enable consistent encoding, schema generation and transport of metamodel-based data. The standard specifies rules for producing XML Schema Definitions (XSD) from MOF metamodels, encoding/decoding MOF metadata as XML documents, and importing legacy DTDs into MOF models.

Key Topics and Requirements

• XMI Schema Design Principles: guidance for producing well-formed XMI Schemas, required XML declarations, namespace use, and design principles to ensure interoperable XML Schemas and documents.
• XML Schema (XSD) Production Rules: rules for transforming MOF-based metamodels into XML Schemas that validate XMI documents.
• XML Document Production Rules: conventions and EBNF-based rules for encoding MOF metadata into XML documents (object structure, references, attributes, packages).
• Linking and References: standardized attributes and mechanisms for identifying elements, linking objects across documents, and handling containment.
• Metadata Differences & Versioning: approaches to express adds/changes/deletes and version attributes for interoperability between tools and model versions.
• Reverse Mapping: production rules for importing DTDs/XML Schemas back into MOF-based metamodels and XML-to-MOF transformations.
• XML Schema Model: XMI representation of XML Schema constructs (types, elements, attributes, groups, keys) to support schema-level interchange.

Applications and Who Uses It

ISO/IEC 19503:2005 is used by:

• Modeling tool vendors (UML editors, CASE tools) to export/import models reliably.
• Enterprise architects and systems integrators for metadata exchange between design and repository tools.
• Repository and metadata framework developers who implement MOF-based stores and need XML interchange formats.
• DevOps and toolchain integrators to automate model synchronization across distributed development environments. Practical uses include model migration, tool interoperability, schema generation from metamodels, automated code generation workflows, and exchange of model fragments between teams.

Related Standards

• ISO/IEC 19501 - Unified Modeling Language (UML)
• ISO/IEC 19502 - Meta Object Facility (MOF)
• W3C XML / XML Schema specifications

ISO/IEC 19503:2005 (XMI) is a foundational standard for metadata interoperability, enabling consistent, validated exchange of metamodel-driven metadata across tools and platforms in distributed development environments.

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 19503
First edition
2005-11-01
Information technology — XML Metadata
Interchange (XMI)
Technologies de l'information — Échange de métadonnées XML (XMI)

Reference number
©
ISO/IEC 2005
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ii © ISO/IEC 2005 – All rights reserved

Contents
Foreword .viii
Introduction . ix
1 Scope .1
2 Normative references .1
2.1 Identical Recommendations | International Standards . 1
2.2 International Standards . 1
3 Abbreviations .2
4 XMI Schema Design Principles .3
4.1 Purpose . 3
4.2 Use of XML Schemas . 3
4.2.1 XML Validation of XMI documents . 4
4.2.2 Requirements for XMI Schemas . 4
4.3 Basic Principles . 4
4.3.1 Required XML Declarations . 4
4.3.2 Metamodel Class Representation . 5
4.3.3 Metamodel Extension Mechanism . 5
4.4 XMI Schema and Document Structure . 5
4.5 XMI Model . 6
4.5.1 XML Schema for the XMI Model . 6
4.5.2 XMI Model Classes . 6
4.5.3 XMI. 8
4.5.4 Extension . 9
4.5.5 Documentation. 10
4.5.6 Add, Replace, and Delete . 10
4.6 XMI Attributes . 12
4.6.1 Element Identification Attributes . 12
4.6.2 Linking Attributes. 13
4.6.3 Version Attribute . 14
4.6.4 Type Attribute . 14
4.7 XMI Type . 14
4.8 Metamodel Class Specification . 15
4.8.1 Namespace Qualified XML Element Names. 15
4.8.2 Metamodel Multiplicities . 15
4.8.3 Class Specification . 16
4.8.4 Attribute Specification . 16
4.8.5 Reference Specification . 18
4.8.6 Containment Specification . 18
4.8.7 Inheritance Specification . 18
4.8.8 Derived Information . 18
4.9 Transmitting Incomplete Metadata . 18
4.9.1 Interchange of Model Fragments . 19
4.9.2 XMI Encoding. 19
4.9.3 Example . 19
© ISO/IEC 2005 – All rights reserved iii

4.10 Linking . 19
4.10.1 Design Principles . 19
4.10.2 Linking . 20
4.10.3 Example from UML . 22
4.11 Tailoring Schema Production . 23
4.11.1 XMI Tag Values. 23
4.11.2 Tag Value Constraints . 24
4.11.3 Scope .25
4.11.4 XML element vs XML attribute . 25
4.11.5 UML profile for XML and XMI . 25
4.11.6 Effects on Document Production . 26
4.11.7 Summary of XMI Tag Scope and Affect . 27
4.12 Transmitting Metadata Differences . 28
4.12.1 Definitions . 29
4.12.2 Differences . 29
4.12.3 XMI Encoding . 29
4.12.4 Example . 30
4.13 Document Exchange with Multiple Tools . 31
4.13.1 Definitions . 31
4.13.2 Procedures . 32
4.13.3 Example . 32
4.14 General Datatype Mechanism . 33
5 XML Schema Production.35
5.1 Purpose . 35
5.2 XMI Version 2 Schemas. 35
5.2.1 EBNF . 35
5.2.2 Fixed Schema Declarations . 43
5.2.3 Schema Production Rules for Non-Primitive Data . 48
6 XML Document Production .49
6.1 Purpose . 49
6.2 Introduction . 49
6.3 EBNF Rules Representation . 49
6.3.1 Overall Document Structure . 50
6.3.2 Overall Content Structure . 51
6.3.3 Object Structure . 52
6.3.4 References. 55
6.3.5 Object Contents . 57
6.3.6 Packages . 59
6.3.7 Attributes . 59
6.3.8 Other Types of Links . 60
6.4 Additional Examples . 60
6.4.1 Inheritance . 60
6.4.2 Nested Packages . 61
6.4.3 Derived Types and References . 62
6.5 Document Production Rules for Non-Primitive Data . 63
6.5.1 Structure Type . 63
6.5.2 Enumeration Type. 63
6.5.3 Alias Type . 63
iv © ISO/IEC 2005 – All rights reserved

6.5.4 Collection Type . 63
7 Production of MOF from XML. 65
7.1 Introduction . 65
7.2 DTD to MOF. 65
7.3 XML to MOF . 66
7.4 XML Schema to MOF . 67
8 XML Schema Model .71
8.1 Introduction . 71
8.2 XML Schema Structures . 71
8.2.1 XSDSchema . 83
8.2.2 XSDAttribute . 83
8.2.3 XSDElementRef . 83
8.2.4 XSDAttributeGroup . 83
8.2.5 XSDAttributeGroupRef . 83
8.2.6 XSDType . 83
8.2.7 XSDBuiltInType . 84
8.2.8 XSDComplexType . 84
8.2.9 XSDComplexTypeContent . 84
8.2.10 XSDSchemaContent . 84
8.2.11 XSDElement . 84
8.2.12 XSDSimpleBase . 85
8.2.13 XSDPattern . 85
8.2.14 XSDEnumeration . 85
8.2.15 XSDInclude . 85
8.2.16 XSDImport . 85
8.2.17 XSDGroup . 85
8.2.18 XSDGroupKind . 86
8.2.19 XSDGroupScope . 86
8.2.20 XSDGroupContent . 86
8.2.21 XSDGroupRef . 86
8.2.22 XSDKey . 86
8.2.23 XSDKeyRef. 86
8.2.24 XSDUnique . 86
8.2.25 XSDUniqueContent. 87
8.2.26 XSDSelector . 87
8.2.27 XSDField . 87
8.2.28 XSDObject . 87
8.2.29 XSDAnnotatedElement . 87
8.2.30 XSDDocumentation. 87
8.2.31 XSDAppInfo . 87
8.2.32 XSDAnnotation . 87
8.2.33 XSDSimpleContent . 88
8.2.34 XSDComplexContent . 88
8.2.35 XSDSimpleComplex . 88
8.2.36 XSDSimpleTypeContent . 88
8.2.37 XSDSimpleRestrict. 88
8.2.38 XSDSimpleList . 88
8.2.39 XSDSimpleUnion . 88
© ISO/IEC 2005 – All rights reserved v

8.2.40 XSDSimpleType . 89
8.2.41 XSDFacet . 89
8.2.42 XSDLength . 89
8.2.43 XSDMinLength . 89
8.2.44 XSDMaxLength . 89
8.2.45 XSDMinInclusive . 89
8.2.46 XSDMaxInclusive . 89
8.2.47 XSDMinExclusive. 90
8.2.48 XSDMaxExclusive . 90
8.2.49 XSDTotalDigits . 90
8.2.50 XSDFractionDigits . 90
8.2.51 XSDWhiteSpace . 90
8.2.52 XSDAny. 90
8.2.53 XSDAnyAttribute . 90
8.2.54 XSDAttributeRef . 91
8.2.55 XSDNamedElement . 91
8.2.56 XSDOccurs . 91
8.2.57 XSDTopLevelAttrbute . 91
8.2.58 XSDTopLevelElement . 91
8.3 XML Schema Simple Datatypes. 91
8.3.1 XSDDate . 95
8.3.2 XSDDecimal . 95
8.3.3 XSDDecimalType . 95
8.3.4 XSDDouble . 96
8.3.5 XSDCentury . 96
8.3.6 XSDBinary. 96
8.3.7 XSDBinaryType . 96
8.3.8 XSDBooleanType. 96
8.3.9 XSDBoolean. 96
8.3.10 XSDByte . 96
8.3.11 XSDDoubleType . 97
8.3.12 XSDFloat. 97
8.3.13 XSDFloatType. 97
8.3.14 XSDInt .97
8.3.15 XSDInteger . 97
8.3.16 XSDCDATA . 97
8.3.17 XSDID . 97
8.3.18 XSDIDREF . 97
8.3.19 XSDIDREFS . 98
8.3.20 XSDListType . 98
8.3.21 XSDList . 98
8.3.22 XSDLong . 98
8.3.23 XSDMonth . 98
8.3.24 XSDName . 98
8.3.25 XSDNCName . 98
8.3.26 XSDNegativeInteger . 98
8.3.27 XSDNMTOKEN. 99
8.3.28 XSDNonNegativeInteger. 99
8.3.29 XSDNonPositiveInteger . 99
8.3.30 XSDPositiveInteger . 99
8.3.31 XSDQName . 99
vi © ISO/IEC 2005 – All rights reserved

8.3.32 XSDQNameType . 99
8.3.33 XSDRecurringDate . 99
8.3.34 XSDRecurringDay . 100
8.3.35 XSDRecurringDuration . 100
8.3.36 XSDRecurringDurationType . 100
8.3.37 XSDShort . 100
8.3.38 XSDToken . 100
8.3.39 XSDString . 100
8.3.40 XSDStringType . 100
8.3.41 XSDTime . 101
8.3.42 XSDTimeDuration . 101
8.3.43 XSDTimeDurationType . 101
8.3.44 XSDTimeInstant . 101
8.3.45 XSDTimePeriod . 101
8.3.46 XSDUnionType . 101
8.3.47 XSDUnsignedByte . 101
8.3.48 XSDUnsignedInt . 102
8.3.49 XSDUnsignedLong . 102
8.3.50 XSDUnsignedShort . 102
8.3.51 XSDURIReference . 102
8.3.52 XSDURIReferenceType . 102
8.3.53 XSDValueConstraint . 102
8.3.54 XSDYear . 102
8.3.55 XSDDecimalRange . 103
8.3.56 XSDIntegerRange . 103
8.3.57 XSDPatterned . 103
Annex A (normative) Conformance Issues .105
Annex B (normative) References .107
Annex C (informative) Legal Information .109
Index.113

© ISO/IEC 2005 – All rights reserved vii

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. In the field of information
technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as
an International Standard requires approval by at least 75 % of the national bodies casting a vote.
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.
ISO/IEC 19503 was prepared by the Object Mangement Group (OMG) and was adopted, under the PAS
procedure, by Joint Technical Committee ISO/IEC JTC 1, Information technology, in parallel with its approval
by national bodies of ISO and IEC.
ISO/IEC 19503 is related to
⎯ ISO/IEC 19501, Information technology — Open Distributed Processing — Unified Modeling Language
(UML) Version 1.4.2
⎯ ISO/IEC 19502, Information technology — Meta Object Facility (MOF)

viii © ISO/IEC 2005 – All rights reserved

Introduction
The main purpose of this International Standard (XML) is to enable easy interchange of metadata between application
development lifecycle tools (such as modeling tools based on the Unified Modeling Language (UML), ISO/IEC 19501, and
metadata repositories/frameworks based on the Meta Object Facility (MOF), ISO/IEC 19502) in distributed heterogeneous
environments. This International Standard integrates three key industry standards:
• XML - eXtensible Markup Language, a W3C standard.
• UML - Unified Modeling Language, an OMG modeling specification, which is now ISO/IEC 19501.
• MOF - Meta Object Facility (ISO/IEC 19502).
The OMG adopted the XMI (version 1.0) in February 1999. It was developed as a response to a request for proposal,
issued by the OMG Analysis and Design Task Force, for a model and metadata interchange facility. The purpose of the
facility was to support the interchange of metadata (such as ODP UML models). The most recent revision of XMI, 2.0,
was submitted by the XMI Revision Task Force in October, 2002, and includes corrections and clarifications to the
original specification, and changes to accommodate revisions to the 1.4 version of MOF.
The rapid growth of distributed processing has led to a need for a coordinating framework for this standardization and
ITU-T Recommendations X.901-904 | ISO/IEC 10746, Open Distributed Processing — Reference Model (RM-ODP)
provides such a framework. It defines an architecture in which support of distribution, interoperability, and portability can
be integrated. RM-ODP Part 2 (ISO/IEC 10746-2) defines the foundational concepts and modeling framework for
describing distributed systems. RM-ODP Part 3 (ISO/IEC 10746-3) specifies a generic architecture of open distributed
systems, expressed using the foundational concepts and framework defined in Part 2.
While not limited to this context, this International Standard is relevant to work on the standardization of Open
Distributed Processing (ODP).
© ISO/IEC 2005 – All rights reserved ix

INTERNATIONAL STANDARD ISO/IEC 19503:2005(E)

Information technology — XML Metadata Interchange (XMI)

1 Scope
This International Standard provides specifications for:

a. A set of XML Schema Definitions (XSD) production rules for transforming MOF based metamodels into XML

Schemas.
b. A set of XML Document production rules for encoding and decoding MOF based metadata.
c. Design principles for XMI based Schemas and XML documents.
d. A set of production rules for importing XML DTDs to a MOF based metamodel.

This International Standard enhances metadata management and metadata interoperability in distributed object

environments in general and in distributed development environments in particular. While this International Standard
addresses stream based metadata interoperability in the object analysis and design domain, XMI (in part because it is

MOF based) is equally applicable to metadata in many other domains.

2  Normative references
The following referenced documents are indispensable for the application 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.
2.1  Identical Recommendations | International Standards
• ITU-T Recommendation X.902 (1996) | ISO/IEC 10746-2:1996, Information technology — Open Distributed
Processing — Reference Model: Foundations
• ITU-T Recommendation X.903 (1996) | ISO/IEC 10746-3:1996, Information technology — Open Distributed
Processing — Reference Model: Architecture
2.2  International Standards
• ISO/IEC 10646:2003, Information technology — Universal Multiple-Octet Coded Character Set (UCS)
• ISO/IEC 19501, Information technology — Open Distributed Processing — Unified Modeling Language (UML)
Version 1.4.2
• ISO/IEC 19502, Information technology — Meta Object Facility (MOF)
• W3C XML 1.0 : http://www.w3.org/TR/REC-xml – February, 2004
• W3C XSD 1.0 http://www.w3.org/TR/xmlschema-0/, xmlschema-1, xmlschema-2
© ISO/IEC 2005 – All rights reserved 1

3 Abbreviations
DTD Document Type Definition
MOF Meta Object Facility
UML Unified Modeling Language
XMI XML Metadata Interchange
XSD XML Schema Definition
2        © ISO/IEC 2005 - All rights reserved

4 XMI Schema Design Principles
4.1 Purpose
This Clause contains a description of the XML Schemas that may be used with the XMI specification to allow some
metamodel information to be verified through XML validation. The use of schemas in XMI is described first, followed by
a brief description of some basic principles, which includes a short description of each XML attribute and XML element
defined by XMI. Those descriptions are followed by more complete descriptions that provide examples illustrating the
motivation for the XMI schema design in the areas of metamodel class specification, transmitting incomplete metadata,
linking, tailoring schema production, transmitting metadata differences, and exchanging documents between tools.
It is possible to define how to automatically generate a schema from the MOF metamodel to represent any MOF-
compliant metamodel. That definition is presented in Clause 5.
This Clause describes XMI 2.0 schemas; Clause 5 describes how to create XMI 2.0 schemas.
You may specify tag value pairs as part of the MOF metamodel to tailor the schemas that are generated, but you are not
required to do so. Using these tag value pairs requires some knowledge of XML schemas, but the schemas that are
produced might perform more validation than the default schemas. See Clause 7 for a complete description of how to
generate XML schemas using these tag value pairs. Sub clause 4.11, “Tailoring Schema Production,” on page 23 describes
the tag values, their affect on schema production, and their impact on document serialization.
4.2 Use of XML Schemas
An XML schema provides a means by which an XML processor can validate the syntax and some of the semantics of an
XML document. This specification provides rules by which a schema can be generated for any valid XMI-transmissible
MOF-based metamodel. However, the use of schemas is optional; an XML document need not reference a schema, even
if one exists. The resulting document can be processed more quickly, at the cost of some loss of confidence in the quality
of the document.
It can be advantageous to perform XML validation on the XML document containing MOF metamodel data. If XML
validation is performed, any XML processor can perform some verification, relieving import/export programs of the
burden of performing these checks. It is expected that the software program that performs verification will not be able to
rely solely on XML validation for all of the verification, however, since XML validation does not perform all of the
verification that could be done.
Each XML document that contains metamodel data conforming to this specification contains: XML elements that are
required by this specification, XML elements that contain data that conform to a metamodel, and, optionally, XML
elements that contain metadata that represent extensions of the metamodel. Metamodels are explicitly identified in XML
elements required by this specification. Some metamodel information can also be encoded in an XML schema.
Performing XML validation provides useful checking of the XML elements that contain metadata about the information
transferred, the transfer information itself, and any extensions to the metamodel.
The XML Namespace specification has been adopted by the W3C, allowing XMI to use multiple metamodels at the same
time. XML schema validation works with XML namespaces, so you can choose your own namespace prefixes in an XML
document and use a schema to validate it. The namespace URIs, not the namespace prefixes, are used to identify which
schemas to use to validate an XML document.
© ISO/IEC 2005 - All rights reserved    3

4.2.1 XML Validation of XMI documents
XML validation can determine whether the XML elements required by this specification are present in the XML
document containing metamodel data, whether XML attributes that are required in these XML elements have values for
them, and whether some of the values are correct.
XML validation can also perform some verification that the metamodel data conforms to a metamodel. Although some
checking can be done, it is impossible to rely solely on XML validation to verify that the information transferred satisfies
all of a metamodel’s semantic constraints. Complete verification cannot be done through XML validation because it is not
currently possible to specify all of the semantic constraints for a metamodel in an XML schema, and the rules for
automatic generation of a schema preclude the use of semantic constraints that could be encoded in a schema manually,
but cannot be automatically encoded.
Finally, XML validation can be used to validate extensions to the metamodel, because extensions must be represented as
elements; if those elements are defined in a schema, the schema can be used to verify the elements.
4.2.2 Requirements for XMI Schemas
Each schema used by XMI must satisfy the following requirements:
• All XML elements and attributes defined by the XMI specification must be imported in the schema. They cannot be
put directly in the schema itself, since there is only one target namespace per schema.
• Metamodel constructs have corresponding element declarations, and may have an XML attribute declaration, as
described below. In addition, some constructs also have a complexType declaration. The declarations may utilize
groups, attribute groups, and types, as described below.
• Any XML elements that represent extensions to the metamodel may be declared in a schema, although it is not
necessary to do so.
By default, XMI schemas allow incomplete metadata to be transmitted, but you can enforce the lower bound of
multiplicities if you wish. See 4.9, “Transmitting Incomplete Metadata,” on page 18 for further details.
4.3 Basic Principles
This sub clause discusses the basic organization of an XML schema for XMI. Detailed information about each of these
topics is included later in this Clause.
4.3.1 Required XML Declarations
This specification requires that XML element declarations, types, attributes, and attribute groups be included in schemas
to enable XML validation of metadata that conforms to this specification. Some of these XML elements contain metadata
about the metadata to be transferred. For example, the identity of the metamodel associated with the metadata, the tool
that generated the metadata, whether the metadata has been verified, etc.
All XML elements defined by this specification are in the namespace “http://www.omg.org/XMI.” The XML namespace
mechanism can be used to avoid name conflicts between the XMI elements and the XML elements from your MOF
models.
4        © ISO/IEC 2005 - All rights reserved

In addition to required XML element declarations, there are some attributes that must be defined according to this
specification. Every XML element that corresponds to a metamodel class must have XML attributes that enable the XML
element to act as a proxy for a local or remote XML element. These attributes are used to associate an XML element with
another XML element. There are also other required attributes to let you put data in XML attributes rather than XML
elements. You may customize the declarations using MOF tag values.
4.3.2 Metamodel Class Representation
Every metamodel class is represented in the schema by an XML element whose name is the class name, as well as a
complexType whose name is the class name. The declaration of the type lists the attributes of the class; references to
association ends relating to the class; and the classes that this class contains, either explicitly or through composition
associations. By default, the content models of XML elements corresponding to metamodel classes do not impose an
order on the attributes and references.
By default, XMI allows you to serialize features using either XML elements or XML attributes; however, XMI allows you
to specify how to serialize them if you wish. Containment references and multivalued attributes always are serialized
using XML elements.
4.3.3 Metamodel Extension Mechanism
Every XMI schema contains a mechanism for extending a metamodel class. Zero or more extension elements are
included in the content model of each class. These extension elements have a content model of ANY, allowing
considerable freedom in the nature of the extensions. The processContents attribute is lax, which means that processors
will validate the elements in the extension if a schema is available for them, but will not report an error if there is no
schema for them. In addition, the top level XMI
...