I just got a book that has S as a notation. At first I thought it would be slap, but Tb is slap in that book. The S is usually an octave of the Tb, but I dont have any idea what it can be.
Well, I highly doubt it's pop, because S...well wouldn't they just put P? Besides, I'm pretty sure the music isn't popped.
It's not slide because the / or \ is slide. I cant figure it out.