C2MS12 — In message tables, make use of an enum type to designate a discreet list

Many fields in many messages contain value information. Some of the values are defaults and some are intended to be constant. Also, this information is in the "notes" column. There should be a way to differentiate between fixed and default values more easily than reading the notes fields. Also, this will make encoding those attributes in the .xsd PSM easier.

Note too, that another issue identified that when the PRIORITY field is optional, it should have a default value. RTF in 12/24 determined it should be medium for the following tables:
Directive Request Message
Simple Service Request Message