Shot placement, penetration and wound diameter.
There are many ways to accomplish this. Velocity along with bullet weight will give a mathematical energy number. It is one way to predict penetration and expansion, but is not perfect. It doesnot take into consideration bullet construction.
I place more stock in ballistic geletian test results and actual street performance. The numbers are not useless though. If you know that load "A" is a proven performer, then you should be able to predict that a different chambering with similar bullets and energy numbers should perform about the same.