It seems to me that the ‘Manage Folders’ → - ‘Unify’ settings is confusing. Let me try to explain.
1) Unify differs from but puts one in mind of, and is indeed related to, the Unified Inbox. The ‘unify’ setting is per account and indeed per ‘folder’. Yet, that setting sounds like the ‘Unified inbox’ setting; and that latter setting is cross-account and applies only to the inbox ‘folder’. Still, or also: the ‘Unify’ setting does have to do with an inbox. For, what the Unify setting does is - yes? - the following. It changes the ‘Unified Inbox’ view such that is shows, not only the contents of all the inboxes of all accounts, but also the contents of whatever account-specific folder than one has just clicked ‘Unify’ on.
2) Unifying a folder has results that can confound. Suppose that you enable ‘unify’ for the Trash folder of some given account. Suppose further that subsequently you delete an email from that account. The result will be this: the email at issue will disappear from and then reappear in one’s unified inbox. Similar strange things happen if one enables ‘unify’ for the ‘drafts’ folder or for the Sent Mail folder. Now I suppose that all these effects are by design - they are the Unifications for which one has asked - but if one was struggling to understand the Unification concept then one will be bamboozled.
Here are two further points. Each is about Unification only tangentially.
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The only ‘folder’ that shows up in my Gmail account - when viewed through K9 - is ‘outbox’.
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When staring at the legend Gmail ‘outbox’, one does see, near that legend, this other legend: ‘unified inbox’. Thus one can get the strange impression that one folder within Gmail is a . . unified inbox.