Well i can be explained like this:

Travel-classThe class you are seated in when you get in the plane, the two normal are economy (M or Y) and Business (C). It can also depending on airline be First (F). On some airline where they have ecnomy extra/plus it usually is shown as (Y) on boardingstubbs.

This is normally printed on boardingstubbs and has nothing to do with the booking-class. You can be booked in the lowest SAS-class wich would be T-class and still have Y-class printed on your boardingstubb..it show that you are seated within the economy-cabin.

Booking-class
This is the class in wich the ticket is booked in the system at the airline or the travel agency. With in Travel-class M/Y there can be like 15 different booking-classes like M, W, W, H, U and so on..

The earnings are based on booking-class