A is for Attack Cruiser, a class of combat vessel in some fleets and space forces. The Vor'cha-class attack cruiser was a Klingon example.
B is for Baroner. Pseudonym used by Jim Kirk on Organia.
C is for Commodore, a rank that fell out of favor in Starfleet by the 24th century.
D is for Dunsel.
E is for Earth Colonies. There were a nice handful out in space by the time the NX program was launched and the first Enterprise was commissioned.
F is for Federation Funny Farm.