In United's webpage it says that "The miles accrued are determined by the operating airline. Please note that the booking class shown on your ticket may differ from the booking class relevant for mileage accrual."

My questions are:
1. Why are there two different systems of booking classes, i.e., what's the reason for doing this?
2. How can I know the booking class relevant for mileage accrual?

Thanks a lot!