A Class "X" Amplifier is a Class "D" Amplifier with a digitally controlled power supply. It is this highly sophisticated power supply that gives Class "X" an edge in overall efficiency when compared to a Class "D" Amplifier.
Class X is a marketing scam for a class T amplifier (like those in headunits)...MA Audio??? They put out ridiculous amounts of power but sound like as$...really good for SPL comps, but bad for daily driver.
it's essentially a class T amplifier with some changes to teh power supply. It's not really a recognized amplifier class per se, but more a marketing sort of gimmick. It's just a full range digital amplifier, but the companies using this "class X" terminology are pretty low end brands.