Basically, "17aa" and "methyl" mean the same thing when used on this board. Chemically speaking "methyl" is just a CH3 group which can be found in all sorts of compounds (like caffeine, for instance), but when people say "Superdrol is a methyl" they mean that it's methylated at the 17aa position. Many steroids are methylated at the 17aa position to make them orally active (it helps prevent breakdown in liver), but this also generally makes them more liver toxic. As Sinner mentioned, though, the liver toxicity can vary greatly from poisonous (M1T) to not-too-bad (Epi/Havoc).