Simple actually. The GM seaters all have a sliding caliber specific insert that carries the pill in alignment with the seater barrel itself. You advance the case upwards until it contacts the insert, drop in the pill from the top/front and then continue the ram travel upwards, seating the pill. The seat depth is controlled with a micrometer head on top. Everything stays in perfect alignment because the insert is a precision fit in the seater barrel. It's actually such a close fit that it takes an ocassional drop of oil to keep it sliding easily.
You can buy one seater die and change out the inserts and shell holders for every caliber. I get them direct from RCBS but I see Midway has them as well.
I like the micrometer head because I can record the insertion depth, switch to something else and redial the depth and go back to the previous seating depth no issue (so long as you don't move the seater itself) in the press. I tend to find a medium for all my loads, lock the adjustment nut on the seater die and adjust with the micrometer head.
I don't like inserting pills from the bottom side, something I have to do with pistoil reloads. Wish RCBS made a GM seater for handgun loads too.
I think RCBS 'borrowed' the idea from Pacific Tool and Gage but I'm not sure on that.