Originally Posted by OtherDave

Regionals can be used on US Domestic and a few other routes between the US and Central America (it's in the link above).

Globals can be used on basically any Delta ticket (including domestically... but that's a huge waste of a Global cert... but if that cert is about to expire, then why not).

When using certs it matters if there is inventory available. If yes, then you can confirm the upgrade right away. If not, you have to be waitlisted. If you're waitlisted, then your upgrade will clear automatically if inventory becomes available. You can ask to be waitlisted for only a specific segment (as in your example, you'd ask to be waitlisted specifically for the 12 hour flight). If your upgrade clears for the long flight, then you call back in and get added to the waitlist for all the other flights (just to better your odds for a gate upgrade).

Things get complicated when you're trying to get the global upgrades to apply on codeshare flights for partner airlines (KLM, Air France, etc.). Then you need to be in specific fare buckets (although even the cheaper buckets are eligible now). But, you can't "waitlist" per se. If inventory isn't available you have to keep calling back to check for upgrade inventory. I'm not sure how globals work on partner airlines for gate clearing.

I probably didn't get all you questions here... but hopefully is helpful.