A3 Aluminum Oxide Premium blue fired aluminum oxide material with a heat treated polyester backing recommended for universal use and provides rapid stock removal and extremely long life. Z3 - Best On Carbon Steel Premium 100% zirconia aluminum grain with a poly/cotton heat treated backing. Provides fastest grain removal on the market for carbon based steels as well as excellent life. Z-Stainless Zirconia - Best On Stainless Steel Excellent on both stainless and carbon based metals. High performance zirconia offers a third layer of resin "grinding aid" containing lubricants, allowing the zirconia to break down with light pressure and to excel in heat sensitive applications. Provides sharper cutting action and longer life than standard discs.