Many samples in one instrument to make it sound better. Dark piano sounds can sound bad on high notes for example then you make so that they automatically play other (brighter) piano samples as you go. I know how to map like that in OpenMPT but not in Schismtracker.
Anyway listen to how you want things to sound then try to make it like that using all the available effects etc. then you'll find out techniques!
Right--multiple samples in one instrument allows the instrument to "track" better over a larger range. Take a sample, any sample, and then play a really low or really high note. It'll start doing strange things, playing really long or really short, and sounding unnatural. So, if, for example, you want a decent piano sound, you would take multiple samples of the piano at different octaves, and then associate each one of them to the appropriate range in your tracker instrument. The result will be a more natural sounding piano, even if you play really low or high notes on it.
Of course,
sometimes you want a really weird or unnatural sound, but that's another issue.