The grit size of a superabrasive wheel will determine several aspects of how the grinding wheel will perform. The coarser the mesh grit size (60 grit mesh) the more aggressive the wheel will act. The higher the mesh grit size the (400 grit mesh) the better the finish you will get from the wheel. Micron is the opposite of this, where the higher number has a coarser finish and the lower number leaves a better finish.
In both cases the larger the number of particles per carat will leave a better finish. The number of carats of abrasive material is determined by the size of the wheel, the concentration of the wheel, and the grit size.