These are called dual inventory fares and they are used for Basic Economy, Premium Economy, and First Class fares (DL also uses them for Comfort+ fares). They are used by AS, UA, DL, and AA and have been around for many years (not all airlines use them for PE fares -- and DL uses them for TPAC PE fares, but not TATL). The first letter of the fare basis code is a coach class letter, but it is NOT the actual Fare Class of the fare. EF frequently screws up and shows the first letter of fare basis code as the fare class, when it is not. The actual fare class can be found in the routing rules for the fare on EF and this is what will show as the booked class and what will be used for mileage credit. There must be inventory in both the coach bucket that is first letter of fare basis code, and the booking class fare bucket (as well as meeting fare rule requirements) in order to book a particular fare (hence the "dual inventory" name). The second to last letter of the fare basis code is generally used to indicate the cabin of service. Many airlines use a B for Basic Economy, an M for Main Cabin, and an F for First Class, but there can be differences between the airlines on how it is used and they can have multiple letters for the same cabin.