Its a made up marketing phrase.
There are 3 types of audio amps : Class A, Class A/B and Class D. Thats it, all the other 'classes' are just made-up marketing bollocks, or madeup classes to differentiate a new way of building an existing amp topology. For example, a 'Class T' amp (you see them around a lot) is just a Class D amp with a tripath chip and power rail switching
Or to put it another way, there are a few basic engine types. You can modify the engine, but modifying it doesnt make it an entirely different engine. Turboing a piston engine doesnt make it a Star Trek warp drive, and in the same way making a slight modification to a class D amp doesnt make a 'class X' amp or any other class
Extra Info For Nerdy Types : Class B and C amps also exist but they are usually used in industrial applications or for FM radio transmission