NXP

NXP has been a prefix for Federation starship registry numbers since the late 23rd century, and was a development of the NX prefix used by experimental Starfleet vessels.

"NXP" stands for "Naval, eXperimental, Pathfinder (or Prototype)", and refers to a Starfleet ship that is not an active-duty vessel but a mere Technology Demonstrator. A ship like this is built by each shipbuilding company contracted to tender designs for a Starfleet Proposal. The Starfleet Corps of Engineers, Research & Development Division will them put each pathfinder through its trials and decide which one most closely meets the original Proposal, and that Pathfinder vessel will be developed into a proper prototype for the proposed class.

These registry numbers are not assigned in the usual Federation manner of (mostly) sequential numbers. They are assigned based on the year of the Starfleet Propsal that gave rise to their construction. Case in point, the Defiant Pathfinder, built for a 2365 proposal for a vessel capable of fighting the Borg, had the "registry" NXP-2365WP/T.

Thus, a Proposal for a light cruiser in 2279 could give rise to five pathfinder ships, with the registry numbers NXP-2279CL/1 to NXP-2279CL/5.


 * Most of this is just my (logical) cconjecture based ona few discrete data-points. The NXP registry itself comes from the {Star Trek: Deep Space Nine Technical Manual''}.