Split into a refactored NDR (metamodel) and a NDR XML supplement.
Figure out metamodel format and how to do metamodel rules.
Figure out if people will have to do a translation plus two conformance validation passes (metamodel pass, format-specific pass). It's nice being able to easily test rules in Oxygen (or via Saxon) now.
Define the basics
A property is... Explain what kind of fields a property can have. Keep it readable.
Don't just point people to the W3C XML Schema spec. Doesn't work for NIEM JSON, UML, etc.
Can map to these references in the format-specific supplements.
Look at the niem.github.io/reference/concepts definitions and improve on them.