Topper80,
CatIIIB no DH ,the gate will be Alert Height before, if something happen you Go-Around , after you 'll go around only in case of RED Autoland light.
Concept of minima change in aircraft cabability to land (frozen below A.Height). So A point is Cpt discretion (regarding aicraft sistems) , B is controlling for roll-out (75) C is controlling if you 'll enter that part of rwy.
75 is to allow commander to monitor the automatic roll-out phase.