VF — Conformance Clause needs some improvement

Sub-clause 2.1 is clear and understandable, but 2.2 and 2.3 are too vague. It is for example not clear if compliance to 2.2 includes 2.3 or not, and 2.3 need to provide a better definition of collaboration.

Make levels of conformance more explicit

Add clause numbers to references.
2.1 remove exception that "optional" elements need not be included.
2.2 clarify that conformance with notation is not required and that this is a subset of 2.1
2.3 clarify that this is a subset of 2.2 and name the specific classes that must be included for conformance.