Originally Posted by GeorgeBurdell

There are a number of views into the Delta seat inventory system depending on whether you're trying to buy or upgrade, etc. Seats have one or more fare classes assigned to them and these determine what can be done with the seats. Upgrades are a different set of fare classes from paid seats. To my knowledge, the only way to see what's close to the true open inventory in first is to look at the upgrade list in the mobile app. Even before the check-in window opens, it will display what's there even if Delta is blocking seats. There are web pages out there with listings of fare classes and descriptions. Expert Flyer used to have visibility into much more of the available fare classes but Delta locked them down a few years ago.