Understanding Delta fare classes is not straight forward or well understood by the casual flyer. 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 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 revenue fare classes only. Award 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.
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.
Amra says:
So essentially there is no differences between Class (U) or Class (T). So why this class system? What advantage it has for airline, travel agents or for people?
Jon says:
They’re mostly used internally but they do affect other things like companion certificates, multipliers, SkyBonus points, Mileage Upgrade Award eligibility, etc.
Noah says:
The Z class is not a Delta One fare class. I am booked in Z on a 738, which does not have a Delta One class.