NEC 600.3 requires that signs be listed and installed in conformance with that listing, unless otherwise approved by special permission, i.e. by the AHJ. What we see most of the time is a cabinet that has listed lighting fixtures. 600.3(B) exempts outline lighting with a listed lighting system but does not make a reference to cabinet signs. Not sure what the justification is for requiring listing with a simple box sign, but I am looking for some type of bar to set as to when it should be required.