Mark III offers three footswitchable modes of operation: Rhythm 1, Rhythm 2 and Lead.
Rhythm 1 is primarily for playing bright and sparkling clean (although a little crunch is available by running the Volume at 10). You might think of Rhythm 1 as “the Fender mode”.
Rhythm 2 is mainly for crunch chords, chunking metal patterns and some blues (but low settings of the Volume 1 will produce an alternate clean sound that is very fat and warm). Think of Rhythm 2 as “the Marshall mode”.
The third mode is by far the most radical, and its two separate knobs will deliver the most lethal crunch or the smoothest, most vocal sounding lead sustain. This mode is pure MESA-Boogie.