Different inboxes

Is Global Inbox a different thing from Inbox?

If you’re referring to the Finder’s sidebar, no. The Global Inbox’s alias is named Inbox there.

… also, unlike the inboxes of any other database, the Global Inbox is always available – either in the Finder sidebar whether DEVONthink is open or not.

You can use it as a GTD Inbox for example. Usually, I let it sync across my devices too, so you are always up-to-date.