tldr; the handful of sounds/ instrument sounds used.
that’s how I think of it anyway. Kind of a more antiquated MIDI term for saved lists of what samples/hardware get triggered by the composition data. One example is the roland MT-32 with dos games. You could hear a games music through the device but change each instrument. Even make it all drums if you want etc. Then you could save those choices as soundfonts.
“keeping its promise” that’s not exactly how i remember it