Grain size will affect the grinding efficiency and roughness of work piece directly, generally speaking, if grain size is fine, the ground surface of work piece will be less rough, while if grain size is coarse, the removal of work pieces is more. So the coarse grain size is suitable for rough grinding and the fine grit size is proper for finish grinding.

The grain size is usually indicated according to the FEPA standard. Grit size is measured in terms of the rated mesh size of several screens and denotes the approximate particle diameter. You can check it in the references induction in Grit Size Checking List for Different Standards:
http://www.jd-diamond-wheel.com/pic/other/2012-06-04-03-56-212.pdf

http://www.jd-diamond-wheel.com