Delta Global Certificate - Guaranteed or Request?
 

I'm looking to book an international flight that has Premium Select options with the next fare class up as Delta One Suites. If I book a Premium Select seat and use a Global Certificate will it automatically upgrade my ticket to Delta One (Z) or am I put on some upgrade list? If an upgrade list, when would I know if I get selected for an upgrade?

It depends whether there is availability in Z class.
https://www.flyertalk.com/forum/delta-air-lines-skymiles/2067393-using-global-upgrades-delta-gucs-post-2-1-2022-changes.html

There are 5 fares classes for D1 (also used for DL FC fares) -- J/C/D/I/Z. There specifically has to be availability in the lowest fare class (Z) on the flight in order to apply a GUC and upgrade immediately from a Premium Select fare. If there is currently no inventory in Z class on the flight, you will have to waitlist and hope that it opens up at some point. If there are currently seats for sale in Z class on flight, that means there is Z inventory availability. Sometimes, there can be Z inventory open on a flight, but no Z fares available for purchase (due to fare rule requirements on Z fares like advance purchase). ExpertFlyer can show class inventory availability on a flight regardless of what is actually available for purchase.