MUST

Mostly for future reference: I think this is the first time I have seen MUST used in uppercase and not in lowercase in any OASIS TC work product! Congratulations!

Not that it needs to change for this version, but do be aware you can write your requirements, say:

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A communication agreement MUST have an identifier that MUST be unique in the context of two communication partners, and SHOULD be universally unique. At any particular point in time one or multiple agreements MAY be in place between parties exchanging messages.
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As simple declarative sentences:

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A communication agreement has an identifier that is be unique in the context of two communication partners. At any particular point in time parties may have one or more agreements for exchanging messages.
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That requirement is then referenced by a conformance clause that reads in part:

Parties must have a communication agreement as defined in (section ref).

I omitted the universally unique because that isn't meaningful and the MAY, although you could put that in a note, because your specification regulates communication agreements, not the number of agreements than anyone can have.

Moving the MUST to conformance clauses is a very minor point and mostly a question of style. At this point I would suggest you stay with what is familiar but do keep it in mind for future work.