Question about booking class
 

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!

To answer your question, #1 normally applies when purchasing a code-share ticket.
If you buy UA coded, but LH operated flight, for example, your UA booking class is meaningless in terms of earning miles.
For #2, you'd have to contact the operating carrier.

As a potential alternative (say, for when a good deal is in play and you don't want to contact the carrier), you can try to duplicate the pricing on ITA. Click through to see the fare and you can usually determine the booking class.