I think that refers to the gain of the tubes,some dealers select the tubes and refers this to the breakup of the tubes,like groove tubes color designation,hard may be earlier breakup when crank up,medium is a bit more headroom previusly to breakup and soft is a clean more headroom tubes,chosse your taste,if you want clean sound go for soft break up,if you want a bit of crunch in medium volume go for medium and if you love rock crunch creamie overdrive the hard