Different level of ''Criticals''
Basically, normal units will have ''Critical lvl1'' which makes Crit only deal 2x damage.
Classes that depend on Crit (Swordmasters, Berserkers, etc), will have ''Critical lvl2'' which makles Crit deal the current 3x damage
Killer weapons also deal 3x. Killer weapons + crit Classes 4x on crit