What is the difference between IFC release and MVD? Can two MVDs for the same IFC versions can have different schemas?
Until recently the answer was - No. All MVDs for the same IFC versions used to work on one EXPRESS schema and MVDxml was to describe restrictions.
But for IFC 4 RV 1.2 release we have got schemes (EXP files) different.
It uses IfcStrippedOptional to prohibit some optional attributes, for example https://standards.buildingsmart.org/MVD/RELEASE/IFC4/ADD2_TC1/RV1_2/HTML/
Why it was not done in mvdXML?
- Tools generate early-binding API from EXPRESS scheme. Now they will create different libraries for RV and others IFC4 mvds.
- Consequently, applications should build different export/import libraries if use early-binding.
- The construction that looks uncertain and unclear. It leaves the attribute optional but specifies some singular datatype.
- Particularly for this case MVD should require value FALSE rather then $ to be more clear