C2MS11 — Align MESSAGE-CLASS with Usage

C2MS 1.0 defines MESSAGE-CLASS as an optional field (in the C2MS Header, "Generic field for missions to classify their message set to aid message disposition.").

A principal user of C2MS reclassifies MESSAGE-CLASS as a required field with a finite set of defined enum-like values.

In order to help align this user with C2MS, the C2MS MESSAGE-CLASS field should become required. Alternatively, C2MS should define a way for a consumer of the C2MS definitions to require what is to C2MS an optional field with defined values.

No need to change in C2MS

After RTF discussion, there is no need to make this field required in C2MS. Systems that use C2MS can enforce requiring this field through business/domain logic.