[1]

Implementation-Defined
An implementation-defined feature or behavior may vary among model processors; the precise behavior is not specified by this specification but MUST be specified by the implementor of each model processor.
Service Modeling Language, Version 1.1 ( 版) http://www.w3.org/TR/2009/REC-sml-20090512/#implementation_definedService Modeling Language Interchange Format Version 1.1 ( 版) http://www.w3.org/TR/2009/REC-sml-if-20090512/#implementation_defined

[2] XProc: An XML Pipeline Language ( 版) http://www.w3.org/TR/2010/REC-xproc-20100511/#dt-implementation-defined

[3] XQuery 1.0 and XPath 2.0 Formal Semantics (Second Edition) ( ( 版)) http://www.w3.org/TR/2010/REC-xquery-semantics-20101214/#dt-implementation-defined

[4] XQuery 1.0 and XPath 2.0 Data Model (XDM) (Second Edition) ( ( 版)) http://www.w3.org/TR/2010/REC-xpath-datamodel-20101214/#dt-implementation-defined

[5] XSLT 2.0 and XQuery 1.0 Serialization (Second Edition) ( ( 版)) http://www.w3.org/TR/2010/REC-xslt-xquery-serialization-20101214/#impdef

[6] XQuery 1.0: An XML Query Language (Second Edition) ( ( 版)) http://www.w3.org/TR/2010/REC-xquery-20101214/#dt-implementation-defined

[7] XQuery 1.0 and XPath 2.0 Functions and Operators (Second Edition) ( ( 版)) http://www.w3.org/TR/2010/REC-xpath-functions-20101214/#implementation-defined

[8] XML Path Language (XPath) 2.0 (Second Edition) ( ( 版)) http://www.w3.org/TR/2010/REC-xpath20-20101214/#dt-implementation-defined

[9] XQuery and XPath Full Text 1.0 ( ( 版)) http://www.w3.org/TR/2011/REC-xpath-full-text-10-20110317/#dt-implementation-defined

CGI における定義

[10]

'implementation-defined'
The behaviour of the feature may vary from implementation to implementation; a particular implementation must document its behaviour.
RFC 3875 - The Common Gateway Interface (CGI) Version 1.1 ( 版) http://tools.ietf.org/html/rfc3875#page-5

関連

[11] CGI では実装定義の他にシステム定義があります。システム定義は OS ごとの定義であり、実装を跨って共通のものです。逆に同じ実装でも動作する OS により異なるかもしれません。

[12] W3C XML Schema Definition Language (XSD) 1.1 Part 2: Datatypes ( ( 版)) http://www.w3.org/TR/2012/REC-xmlschema11-2-20120405/#key-impl-def

[13] Web Services Internationalization (WS-I18N) ( ( 版)) http://www.w3.org/TR/2012/NOTE-ws-i18n-20120522/#dt-implementation-defined

[14] XML processor profiles ( ( 版)) http://www.w3.org/TR/2014/NOTE-xml-proc-profiles-20140206/#dt-implementation-defined

[15] XQuery 3.0: An XML Query Language ( ( 版)) http://www.w3.org/TR/xquery-30/#dt-implementation-defined

[16] XML Path Language (XPath) 3.0 ( ( 版)) http://www.w3.org/TR/xpath-30/#dt-implementation-defined

[17] XQuery and XPath Data Model 3.0 ( ( 版)) http://www.w3.org/TR/xpath-datamodel-3/#dt-implementation-defined

[18] XPath and XQuery Functions and Operators 3.0 ( ( 版)) http://www.w3.org/TR/xpath-functions-3/#implementation-defined

[19] XSLT and XQuery Serialization 3.0 ( ( 版)) http://www.w3.org/TR/xslt-xquery-serialization-3/#impdef

[20] XQuery and XPath Full Text 3.0 ( 版) http://www.w3.org/TR/2015/REC-xpath-full-text-30-20151124/#dt-implementation-defined

Implementation-defined indicates an aspect that may differ between implementations, but must be specified by the implementor for each particular implementation.

[21] XQuery 3.1: An XML Query Language () https://www.w3.org/TR/2017/REC-xquery-31-20170321/#dt-implementation-defined

[22] XML Path Language (XPath) 3.1 () https://www.w3.org/TR/2017/REC-xpath-31-20170321/#dt-implementation-defined

[23] XQuery and XPath Data Model 3.1 () https://www.w3.org/TR/2017/REC-xpath-datamodel-31-20170321/#dt-implementation-defined

[24] XPath and XQuery Functions and Operators 3.1 () https://www.w3.org/TR/2017/REC-xpath-functions-31-20170321/#implementation-defined

[25] XSLT and XQuery Serialization 3.1 () https://www.w3.org/TR/2017/REC-xslt-xquery-serialization-31-20170321/#impdef

[26] XSL Transformations (XSLT) Version 3.0 () https://www.w3.org/TR/2017/REC-xslt-30-20170608/#dt-implementation-defined