I've spent some time on various sources of fare class information, but what puzzles me, is that that some of the fare classes in use seem to be redundant.

For example on an up coming trip a I have "I" on ATL-SCL, and "C" on the return of SCL-ATL. It looks like the MQM earnings is the same, and I can't find any other differences.

What are the differences? Where can that information be found? And if there are no differences, why does Delta use redundant fare classes?