Many Eventers practice at home using Minute Markers to gage their speed so that when they are on course at a comp, they don't have to rely so much on their watches, when instead they can go back to their horses speed on course, to decipher who fast or how slow they are going.
The reason this is done, is so that Eventers stop riding their watches, and start riding their horses on course, to help prevent rotaional falls that occur so often.
So, yes - MPM is a very well known and common abbreviation used amongst Eventers. When a Coach or an HT Offical says "your course is 450 MPM", an Eventer SHOULD know what that means! If they don't, they need to go home and set up a long gallop stretch with minute markers to work on it, so it becomes second nature.