In Apple Mail in MacOS High Sierra I move all old sent and received mails out of the IMAP folders into a folder that is displayed as "Local" in the sidebar:
That way I archive all my old mails and remove them from the IMAP folders from the internet. Whenever I get a new Mac, I export those local folders and import them back into Mail on the new Mac. So my archive stays complete and searchable.
But Apple also has this thing called "Archive":
Which is not displayed under "Local" but its content is actually stored under local.
So what is the actual functional difference between the two, except from them having different icons?