In theory upgrades are prioritized by a combination of fare basis and medallion status. That said, there is no transparency to the process, and no data that you can access that can prove that it is actually being followed. Delta has been actively "monetizing" First class for increased revenue. Can't fault them for that, but it does create less and less seats that are available to upgrade to.

The challenge with FCM, and battlefield (gate) upgrades is that there seems to be a somewhat random methodology of where you end up on upgrade list. Again, because there is no data or transparency of their process, you are left without actually knowing if they have followed the process or not. Many a time as a diamond, I've seen gold medallions up front while I head to back cabin. Yes, perhaps they were on a paid ticket, or a Y fare, or used some other way to get there, but we will never know because we can't see the data.

Now, please note that there are "flavors" of "Y" tickets that will qualify as a Y ticket for upgrade, yet are substantially cheaper than a regular Y ticket. Example: government fares, DOD, and more. Also, there are the unknowns of the "white envelope" fliers that DL treats very special. I'm certain those groups of people have ways around the upgrade list.

All that said, the pecking order has been covered many times but it is supposed to follow this:

IRROPS
UP2A -DM desiring to use SWU
UP2B -PM desiring to use SWU
UP2 -Full Y medallion desiring comp upgrade
UP3D -DM desiring comp upgrade (including award tickets)
UP3 -PM desiring comp upgrade
UP4 -GM desiring comp upgrade
UP5 -FO desiring comp upgrade
UP6 -Companion or AS/AF/KL elite desiring comp upgrade
UP7 -Psgr on award ticket booked in R/O and confirmed in coach
SBUP- Delta paid standby upgrade program

And then there is the Delta Reserve AMEX which is good for tie breakers.