Maybe it stands for "SUBSTITUTION"? I don't know for sure but that's basically what you can use it for. Substitute in one pattern for another instantly, instead of waiting for the pattern to end?

At first i thought it had to do with Subsonic bass..

Yeah, that's what I thought originally.

I guess substitute makes sense... all it really is is a "soft" pad, really only good for hitting a sample faster than you can physically hit one pad.
SUB could also mean "below" as in below the other pads, a slave pad.

another use for it is for double hits, I think somebody already said that though (eg 1 sub 2 sub 1 sub 2 sub)

hi hats.

i tried this a bunch and it can def produce a few desired effects
one thing i found it was real useful for is say you do a pattern and first quantize your kicks on 4/8 then you want to go back in and add more on 16/32 the sub pad is the truth