In light of the prefix variants that are
canonically witnessed, it would seem that the N in them might stand for "Federation". That is, NFT, NGL, NAR, NSP and NCC are all witnessed in connection with Federation vessels, and that's about the only thing in common between them.
It would then appear to follow that "CC" means "Starfleet active service" and "X" means "experimental or prototype"; the other letters don't have such systematic or clear-cut correlation to any observable characteristic...
Two other apparent registry prefixes are BTR for a colony ship in DS9 "Paradise", and YLT for an Yridian craft in "Birthright". The former hauled Federation colonists, but that need not invalidate the "N = Federation" model as such - after all, the latter hauled a Klingon Starfleet officer holding Federation citizenship!