What's the difference between a Main, Supplemental, and Assistance exercise?

Main exercises are the big compound lifts like Squat, Bench, Deadlift, Press, Cleans, etc. 

Supplemental exercises are the same exercises as the main lifts (or a similar compound lift) but done for more reps at a lower weight. 

Assistance exercises are all the accessory exercises like curls, tricep pushdowns, pull-ups, single-leg split squats, etc.

Main and Supplemental exercises are percentage-based and the weights will be automatically calculated for you based on your Training Max. For assistance exercise your input the weight directly.