Tools referencing in ITS Module conformance?

In section 5.9.4 Conformance to the ITS Module Specification, it is indicated in
details what an XLIFF agent must do to be
conformant with the ITS module. There is no mention of the Tools Referencing
mechanism in that section. The requirement to always
support it. It is done only in the section 5.9.5 ITS Tools Referencing, and it's
done in a note, which does not help in establishing
if support is a MUST or a MAY.