XMI13 — Level of required support for XPointer

It's not clear from section 4.9.2 exactly what the set of valid choices is:
it should clarify the intention that any Xlink functionality is available
and this section is offering some guidance. (However start of 4.9 states
that Xlink is not required as a prerequisite).
Support for full XPointer capabilities potentially places too great a
burden on XMI importers (e.g. it could require a repository to support
Xpointer as a query language): some level of optionality should be
specified.

see above