As per Indian Standards the grain size 46 comes under the group "Medium"

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

Resinoid bond is used for speed wheels. Such wheels are used in foundries for dressing castings. Resinoid bond wheels are also used for cutting off. They are strong enough to withstand considerable abuse.
Resinoid Bond is denoted with symbol "B".

Structure indicates the amount of bond present between the individual abrasive grains and the closeness of the individual grains to each other. An open structure wheel will cut more freely. That is, it will remove more metal in a given time and produce less heat. It will not produce such a good finish as a closely structured wheel.