Why Solar X-ray flares are classified by peak soft X-ray flux in the 1-8 angstrom band using the letters A, B, C, M and X, each step being a tenfold increase in intensity. X-class is the top of that scale, with flux at or above 10^-4 W/m^2, and these are the flares most likely to cause sudden ionospheric disturbances and HF blackouts on the sunlit side of Earth. There is no class above X; very large events are simply given higher numbers, like X10 or X20.
Watch out Class M is the tempting pick because M-class flares do get news coverage and can disturb HF, but M is one full decade weaker than X. Class A is the weakest of the real classes, and there is no Class Z at all.
A, B, C, M, X: X marks the extreme. Each letter is 10 times the one before.