While I have no direct experience with these systems as implemented on large aircraft, my understanding with respect to mode S transponders is that:
-on the ground, mode S selective responses should normally be active, and carry coded data including aircraft ident...mode S does some other neat things, including data exchange for TCAS and ADS-B
-modes A and C should not be active on the ground
I'd read that as saying that "Auto" in flight gives you modes A and C as well as S, whereas on the ground it's S only.