ROVAG - Rapid Orbital Ground Assault Vessel is one that I definitely added myself. Basically, some troop transports are designed for work behind the frontlines, and they get a TT designation. Others are designed to airdrop say a corps of troops using drop bays, while wading through STO fire and possibly some weak defensive fleet. Those get the ROVAG designation.
I am also not sure if Laser Assault Frigate and Boarding Shuttle were already there or if I added them myself. Their names are self-explanatory.