Originally Posted by
RealHJ
You need to use GUCs for GUC upgrade inventory (to the last seat…usually). If you want to use RUCs, look for flights where it says RUC space is available (called “upgrade certificate” on the website, without the “global” in front). Be prepared to look and keep looking across many months and keep doing so for weeks until (and if you ever) come across the rarest of unicorns, that is Delta RUC upgrade on A330/350/767. It can happen, but you need to be very lucky to find it.
As for applying it online, IME that never works (it may only work with GUCs). That option just doesn’t appear then at all. Will take a good half an hour for a res agent to do that for you, and they will claim it’s not available, but stand your ground and they should relent and let you have it in the end.
If you get a good agent they can long sell it using a RUC and it'll confirm. I was told by numerous agents that they know system isn't providing the correct inventory. (Ie if GUC shows able to be used for domestic D1, so should RUC, but it's not happening) They stated system will confirm it if agents is willing/knows how to do it.