A typical example is "MeasurementUnitRef" and "MeasurementUnitDef".
As define.xml has been developed as a "vendor extension" to the ODM, I looked into the ODM specification about what it says about vendor extensions.
Section 2.4 of the ODM specification (1.3.1) which is about vendor extensions states:
- The extension may add new XML elements and attributes, but may not render any standard ODM elements or attributes obsolete
- Removing all vendor extensions from an extended ODM file must result in a meaningful and accurate standard ODM file
- Applications that use extended ODM files must also accept standard ODM files
Whether they then do something with the information is another case.
So for me it is clear that the define.xml team is not allowed to forbid the use of ODM elements that are not explicitely mentioned in the define.xml spec, nor state that such elements "are not part of the standard".
If they do so, they break the rules of the ODM standard on which they base their extension.