The theory is since one pad moves and the other doesn't they need to be different formulated pads. One formulated for the outside pad (stationary) and one for the inside pad (pad on the piston). I have a set on my Thunderbird running PBR calipers, they do well. Quite a few of my customers use them and love them. The heavier duty vehicles, mainly one tons and larger, do not have these pads available; they are intended for lighter/sporty vehicles. Oddly though they do have a set listed for my Suburban (8600 GVW).