However, in that series, "NX" is not the designation for an experimental starship; rather, it is a ship class in the same way that starships in "later" series are identified as "Constitution" or "Excelsior" class.
However, in that series, "NX" is not the designation for an experimental starship; rather, it indicates the vessel's class. The NX-01 is an NX class starship in the same way that starships in "later" series are identified as being "Constitution" or "Excelsior" class.