I agree, and am curious for an answer on this too. Attached is a drawing that anyone but ADT programmers could draw a roof from. I've been trying to get my head into this too, but the cryptic B and C dimensions in Properties for roof slabs seems insane! Who draws this way? Nobody I've ever met.
Should just be options for thickness, pitch, plate ht., overhang, and critically - if the roof slab (rafter) starts at the inside face of the wall, outside face, or an offset dimension (horizontal).
I can make no sense of what Autdesk has done here and it's infuriating to waste time trying to understand it their nonsense when it could have been so simple.