Anon said it was 2 months ago
Here are a few that we know about.
Military Callsign Prefixes
RCH#### = 'Regular' transport callsigns, abbreviation of REACH
PAT### = Priority Air Transport
SAM = Special Air Movement or Special Air Mission
SPAR = Special Priority Air Movement
CNV#### = Convoy, used by Navy transports e.g. C-40 (version of Boeing737). Same role as Air Force C-40s to transport primarily personnel and/or light freight
We've also been told that many individual units have their own callsign naming protocols but I don't have that info.
whose?