It's a fuzzy memory, but I saw the breakdown once. They aren't initials, just designators. For road bikes, R for two stroke, X for four, V for V-type motors, Z adds liquid cooling. Going from a fuzzy recollection that's at least 10 years old:
RD - two-stroke road bike
RZ - two-stroke liquid cooled
XS - four-stroke road
XV - four-stroke V (Virago)
XZ - four-stroke liquid cooled V (Vision)
XVZ - liquid cooled V (four cylinder) (Venture)
XJ, FJ - inline air-cooled fours
FZ, FZR - inline fours, air vs liquid cooled.
I sort of lose it with FZR and YZF, then they throw in an M and I'm hosed. Not sure where SR comes in and SRX is another weird one. Probably ran out of designators, or started incorporating major chassis variances.