C+ and BE fares use underlying coach fare class (first letter of fare basis code) as part dual inventory check when booking fares just like FC fares. It's explicit for C+ fares in the fare rules just like FC fares.


They don't explicitly mention it in the BE fare rules, but dual inventory checking is clearly being used when booking BE fares (which only have a single 'E' booking class available), just like C+ and F. PS fares can also have underlying coach fare classes. These mostly seem to be used on TPAC PS fares. TATL PS fares don't typically seem to use underlying coach classes.

All DL metal awards have a secondary letter in the award class, the DL systems just don't always display them. In the flight listings when searching for awards, they just show as N (for both BE and Main), S, R, and O. It's only after you select and display the details of the flights where DL reveals the full award class with secondary letter. BE awards book into "NE" class. Main Cabin awards book into "NV", "NX", "NT", etc., C+ awards book as "SW" or "SN" (I guess they probably wanted to avoid using "SS" here), PS awards book into "RG", "RA", and "RP", and FC/D1 awards book into "OZ", "OI", "OD", "OC", and "OJ". I would expect they likely use the secondary letter of booking class as part of upgrade priority.

In addition to the booking class, there's an underlying coach class embedded in the third letter of award fare basis codes. For main cabin awards, these just match the second letter in the booking class. As an example, the cheapest D1 EU awards from US (with a few exceptions for certain routes) are currently 105K each-way and have a fare basis code of CSVR1050. The 'C' indicates it's a D1 award, the 'V' in the third letter is the underlying coach class, the 'R' indicates a roundtrip booking is a requirement, and the '1050' is just the mileage amount of the award divided by 100. These low-level D1 awards book into "OZ" award booking class.