There are three use cases with this.
1) The Aux track contains NO clips. In this case, "Thru Solo (C)" makes sense.
2) The Aux track contains audio clips and IM is enabled. Solo should work the same as regular audio tracks, AND "Thru Solo(C)" should pass all audio from other sources as well.
3) The Aux track contains audio clips IM is NOT enabled. Solo should work the same as regular audio tracks.