Really, this is just a bunch of notes I'm thinking of as I'm compiling TOS-era ships from various RPGs.. it got me thinking on what the various 'designations' would be, and how they would fit in with the fleet.
Anyway, thoughts welcome.
NCC - Naval Construction Contract. This designation is primarily reserved for active Starfleet registries and ships designed and appropriated primarily for active duty. Example: NCC-1701 is the registry of the Constitution class Enterprise cruiser.
NAR - Naval Auxiliary Registry. This designation is reserved for starships which may be called upon for Starfleet duty, but otherwise operates in an independent manner. Many science ships, support ships, and the merchant marines have their vessels follow this registry pattern.
NX - Naval Experimental. This designation is reserved for engineering test-beds or starship prototypes reserved for trial-runs. In almost all cases, the NX designation is replaced with the NCC designation as soon as the vessel enters active service.
NSP—Naval Scientific Protectorate. These ships are scientific research ships almost exclusively employed by Vulcan. Unlike NAR-designation ships, they may not be called into Starfleet duty without express permission from the Vulcan consulate.
NDT—Naval Diplomatic Transport. Starfleet vessels designed for the shuttling of personnel, diplomats, and passengers over long distances. (This is a recent designation, replacing the NCC designation for personnel transport containers and similar ships.)
NFT—Naval Freight Transport. Starfleet vessels designed for the shuttling of freight and materials over long distances. (This is a recent designation, replacing the NCC designation for freight transport containers and similar ships.)
USS—United Starship, formerly United Spaceship. This designation refers to the christened name of all Federation starships.
SS—Spaceship or Starship. This designation refers to Federation vessels not a part of Starfleet. (It is often used by completely independent vessels operating within Federation space as well.)
(This leaves NGL, which seems to be Naval - Goods and.. something involving freight. I couldn't really tell what the lone ship with this designation was supposed to be, though, other than a frieghter...)