History
Some details to take care of:
- The first edition of ISO 10303-21:1994 had some bugs, which were corrected by a Technical Corrigendum. Therefore, it is recommended that users study the second edition instead (see below).
- The second edition ISO 10303-21:2002, including all the fixes and extensions for several data sections.
- Part 21 defined two conformance classes. They differ only in how to encode complex entity instances.
- Conformance class 1 is always used enforce the so-called internal mapping, which is more compact.
- Conformance class 2, which is not used in practice, always enforces the external mapping. In theory this would allow better AP interoperability, since a postprocessor may know how to handle some supertypes, but may not know the specified subtypes.
- The 1st edition of part 21 enforces the use of so-called SHORT NAMES, which are optional in the 2nd edition. In practice, however, SHORT NAMES are rarely used.
- The 2nd edition allows multiple data sections to be used. In practice, however, most implementations only use a single data section (1st edition encoding).
Read more about this topic: ISO 10303-21
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