This is the amount of training (aka number of sets) that allows you to maintain your current level of muscular size.
Minimum Effective Volume MEV
This is the amount of training that actually grows your muscles: anything below this amount may only maintain them.
Maximum Adaptive Volume MAV
This is the range of volume in which the most optimal muscle gains happen.
Maximum Recoverable Volume MRV
The range of volumes in which you make your best gains. It’s much more of a range than the other volume landmarks because it changes greatly within each training microcycle (week to week).