Some producers, especially those in the film industry, define "stems" as groups of like/similar individual tracks. So a request for stems on a song might include drums and percussion, guitars, keys (piano, organ, synth), horns, etc., etc. in sub-mixed groups, so that a remixer can balance all of the major elements of the music with SFX and Foley, as well as the main dialogue.
At least some rap mixers use the term in the same way, as a stem request might be made so that a vocal can be made to better "fit" with a beat created by another producer, without actually having to re-mix the entire piece from the multi-track mix (every single individual track).
GJ