3M 36166 321U Series Multi-Hole Abrasive Disc, 5 in Dia, 400 Grit, Hook and Loop, Blue

Our 3M™ Hookit™ blue abrasive disc 321U delivers best in class performance at every stage of paint prep and body repair. Premium aluminum oxide mineral abrasive is blended with ceramic grain for a full range of automotive sanding applications. Our multi-hole discs feature a specially-designed pattern that reduces loading, extends disc life and aligns with most backup pads. Hookit™ attachment makes disc changes easy.
3M Hookit hook & loop disc uses ceramic aluminum oxide as the abrasive material and has a diameter of 5 in. 3M incorporates this ceramic aluminum oxide material into the hook & loop disc forming a grit of 400. To aid in dust control, this product is designed to work with vacuum systems and comes with Multi-Hole holes built into the Hook & Loop Disc.
Specification parameters description:

Abrasive Material:
Values: A/O Aluminum Oxide AO A/Z Alumina Zirconia AZ S/C Silicon Carbide SC Aluminum Oxide Aluminum oxide is an artificial abrasive suited for general purpose stock removal and finishing. It is the mostly commonly used abrasive and can be used for most steels and ferrous alloys. Aluminum oxide is slightly softer but tougher than silicon carbide. Ceramic Ceramic is used for aggressive cutting and grinding of stainless steel, titanium, and metal alloys. The abrasive grain is designed to fracture during use so that it maintains sharpness. Ceramic Aluminum Oxide Cerium Oxide Cerium oxide is used for fine polishing and lapping of glass, gemstones, optical mirrors, and lenses. It should be used wet or in slurry form. Diamond Diamond can be either natural or manufactured and is one of the hardest abrasive materials. It is commonly used for grinding and polishing stone and very hard steels, and for sharpening other cutting tools. Nylon Silicon Carbide Silicon carbide is a manufactured material that is extremely hard but brittle. It is commonly used for fast cutting under light pressure, particularly on cast iron, nonferrous metals, and materials such as glass and plastic. Precision Shaped Ceramic Grain Zirconia
Grit:
Grit is a measurement of the size of abrasive particles embedded in coated abrasives. The most common standards for grit size are the United States CAMI (Coated Abrasive Manufacturers Institute) and the European FEPA (Federation of European Producers of Abrasives). A smaller grit number means larger particles and a coarser grade, while high number grits are used for finer work. Grit is generally a unitless measure but is sometimes measured in microns.
Number of Vacuum Holes:
Some sanding discs have a pattern or 5, 6, or 8 vacuum holes, or even a pattern of many smaller holes, to allow a power tool to minimize dust created during operation. Reduced dust can keep the abrasive grains on a disc from clogging and thus can improve disc life. Note that sanding disc backing pads are also sold with the standard hole configurations and should match the sanding disc if they are used.