My understanding: you can route multiple 7.1.2 tracks into a 7.1.2 group, and multiple 7.1.2 group tracks to the final 7.1.2 master bed (defined as Bed in ADM). Its looks like a standard routing.
But for objects, I think it’s different, an Atmos object is a single mono track or a stereo object is a stereo track with spatialization metadatas. So you can also create an object from a stereo group track.