OH I get it now. This is a folded group of "sends," each "send" mixing multiple independent tracks.
You need this plugin in order to create a routable destination from the source tracks into the chains of the midi instrument rack. So it looks like this:
track 1 has say, kick #1 and sends to track 5's "kick mix" chain
track 2, kick #2 -> track 5 "kick mix" chain
track 3, snare #1 -> track 5 "snare mix" chain
track 4, snare #2 -> track 5 "snare mix" chain
then track 5, overall percussion sub mix track, has two sub mixes itself, in the form of an midi instrument rack with two chains, the "kick mix" and the "snare mix".
you can now independently control volume of each sound independently (using the original track volume), the snares together (using the chain volume), the kicks together (using the other chain volume), or the whole kick/snare cluster together (using track 5 main volume).
mkay. nice idea.
now that i get it, i wonder - why is the plugin dll 4mb in size? that's a lot for audio thru. did you use some kind of wrapper application where the base footprint of the plugin has to include the whole platform? maybe someone could cook up a raw, compiled plugin that's much lighter weight? should only need a few kilobytes really.