DD — Interchange extension

The last pagraph of the DiagramElement section says:

Other properties of diagram elements that tools need to interchange
but are not defined in the metamodel can be interchanged using the
extensibility mechanism that is native to the used interchange
format. For example, in an XMIbased interchange, extended data can be
placed on diagram elements within <xmi:extension> tags.

I'm not sure we should be mentioning interchange level extensions,
especially without mentioning metamodel extensions, and domain-specific
extensions like profiles. Can we remove this paragraph, or generalize
it to include other kinds of extension mechanisms?

The specification should not suggest ways of interchanging non-standard information