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

Add Enumerations and Constants as Field Descriptors

Define a way to designate Enumerations and Constants for Fields in C2MS.

Note that this issue resolution also takes care of the default value for PRIORITY. It does this out of convenience of already working on those tables here.

Note, too, that this issue is interdependent on the resolution for C2MS12-8, assumes it is being voted on at the same time.

Because this resolution is connected with PROD-TYPE and PROD-SUBTYPE, through consideration about whether these represent enumerated values, this effort has resulted in analysis of those fields. Upon investigation, there needs to be better documented clarity regarding these and this resolution addresses those issues as well as can be seen in Table 8-155, 8-159, 8-165, and Appendix text and tables.

Additionally, this resolution deals with teh RESPONSE-STATUS field, which is an enumeration. While making these updates, it was discovered that these could use standardized representation across the messages that include RESPONSE-STATUS, so those changes have been made here.