Within the Trek universe, we could say the Federation way of doing it owes something to the old United Earth way of doing it, in the sense of historical homages rather than slavishly identical schemes.
In turn, the UE Starfleet would probably want to follow old Earth naval tradition on most things. Thus, starship classes would be named after the first vessel, even though there could also be parallel and more futuristic designation schemes: hence Archer's ship would begin the Enterprise class (just like ships hundreds of years before and after his), despite also beginning the NX class in the futuristic designation scheme (just like another Enterprise in the real world began the CVN class, and like many other classes like "668 class" or "AEGIS class" have been primarily known by such terms rather than by the classic class names they also possessed).
So Archer's ride could well be NX class "coincidentally", just because it followed the NW class or the MX class chronologically, and not because NX holds some special symbolic meaning. And UFP Starfleet could use NX for experimental vessels not just because it makes general sense (X=experimental is a no-brainer, and it seems UFPSF just plain likes things beginning with N) but also because the admirals would think it's cool how this reminds of Archer's historical ship.
Of course, it's just as possible that NX on Archer's ship meant the exact same as NX on Sisko's ship: an experimental or prototype design. And frankly, the first four or five ships of that design would all have been more or less experimental throughout their operational lives, never warranting another kind of designation.
Let's also remember that it's never quite established that "NX class" would be an in-universe designation for Archer's specific type of vessel. The first place where this expression is used, ENT "Fortunate Son", has Archer call his vessel and the three on the drawing boards "NX class" merely as distinction against what he calls "old freighters". Perhaps NX means something like Naval Explorer, and Archer informs the pirates that they are facing a UE naval exploration vessel this time, an adversary well beyond their powers. And the three other "NX class ships" he refers to might mean three other NX classes rather than three further vessels of the same design as Archer's - after all, that's how a "drawing board" would be required for creating them.
Beyond this, "NX class" is used in "E2" where it seems to more specifically refer to a ship similar to Archer's. It doesn't need to refer to a design identical to Archers even there, though.
The third and last time we hear "NX class" is in "Home", and this time it is actually "the NX class", which finally makes it sound as if it refers specifically to the design flown by Archer. But that could be because the next three NX classes are still in the drawing boards...
Timo Saloniemi
