Satellite A/B/C refers to the 101, 110 and 119 orbital satellite positions.
DirecTV sends most of it's programming on the 101 satellite. It uses 110 and 119 for hi def, Spanish, and some local tv stations.
If you are getting all of your programming off of a single, round, 18" dish, then you are only using satellite A. If you have an oval 3-LNB dish, or if you have two round dishes pointed different directions, then you are using more than one satellite.
If you are using a multi-satellite system, then for the applicable stacker you would have to have four inputs, not just two, and you would not be able to use the internal de-stackers in the D11 and R15.
Carl