That's not what he said. They are 3 different machines.

NSR250 and NSR500 - factory race bikes never sold to public
NSR250R, NSR250R SE and NSR250R SP - street bikes sold to public
RS250 - race bikes with no street equipment sold to privateer racers

The models MC16, MC18, MC21, and MC28 are models associated with the street going machines. Plain R model is the wet clutch, non adjustable suspension version, SE is the dry clutch, adjustable suspension version, and SP is the same as SE but with the addition of lightweight MAGTEK wheels, special paint and limited production (usually around 1000-1500 units).