All of the above, plus: if that's what your mastering engineer is asking for—a "stem" of overheads, kick, snare, bass, et al.—it seems to me he's practically asking to remix your track.
"Id like stems, like this:
Kirk, snare, overheads, toms, guitar 1. Guitar 2, bass, vocal 1, vocal 2, harmonies, and percussion, give me that and I'll master your tracks right up!"

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