Understanding Delta award/miles fare classes is not straight forward since Delta Air Lines switched to dynamic pricing on award tickets. This article was written by a very frequent Delta Air Lines flyer who has spent many years studying how the airline industry operates and documenting the changes as the full-service airlines continually devalue frequent flyer benefits.
This article is designed to help a Silver, Gold, Platinum or Diamond Medallion understand award fare class restrictions and the associated pecking order for upgrades to premium cabins.
The information contained is not endorsed by Delta Air Lines. If you find something incorrect, please send me a message and I’ll get it updated.
Delta Fare Class Groups
Since the 2015 introduction of revenue requirements to the SkyMiles Medallion program, Delta has added TONS of fare classes and made it much more complex to keep track of each. This article addresses award fare classes only. Revenue fare classes and upgrade fare classes can be found in their respective articles.
Fare Class basics
The ultra-simplified way to understand Delta fare classes and how it associates to ticket price, restrictions and upgrade eligibility is this:
Delta One, Delta Premium Select, Comfort+ and Main Cabin all have multiple seats and fare classes available for for sale to customers. By default, you’ll always buy the cheapest fare class available in the cabin of service; however, the less seats available in a fare class causes the fare class (not cabin) to become full. Then the next (more expensive) fare class becomes available for sale (with an increase in ticket price) until the cabin is sold out.
If you don’t know, now you know
Fare classes do not relate to where you sit within your cabin, they are just a placeholder for how much you paid. You can easily purchase a last minute Main Cabin ticket in the Y-fare bucket and sit a middle seat in the last row, while a basic economy ticket may be sitting right behind Comfort+ in a preferred seat – this is due to the economics of supply vs demand (i.e.: less seats in the cabin = higher fare classes, and thus prices).
Once all cabins have been sold out, the flight no longer has availability and is considered sold out.

Upgrades and important rules
The Delta Air Lines official upgrade order and rules are published here which does clearly outline the process that should be followed for upgrades; however there are some caveats that frequent flyers may not know:
Revenue Fare Class Upgrade Order
When you find your fare class, you can expand it and click through to get fare-specific information for your fare class.