As per Indian Standards the 'M' grade of grinding wheel comes under the group

As per Indian Standards the 'M' grade of grinding wheel comes under the group Medium.

The term grade as applied to a grinding wheel refers to the tenacity with which the bond holds the abrasive grains in place, and does not refer to the hardness of the abrasive.
A wheel from which the grains are released rapidly during the grinding operation is called soft grade, whereas wheel that retains the grains more securely is called hard grade