What is a Sub?
A sub or submaster is really just a cue, but rather than going to a look and getting rid of the previous cue, submasters can be stacked upon one another combined in order to create a new look.
So we might have a submaster for house lights and another for stage lights. Both submasters might then be on a physical fader than can be turned up or down.
Submasters can be incredibly helpful when recording or a way to handle the uncertainty of unrehearsed live events.