I also am still confused what these different labels/features do – trying to set up incoming email to use with the Tickets Plugin 
but getting inconsistent behavior.
I feel like the default behavior should be that whatever gets forwarded in becomes a post by the (staged or regular user) that created the message. The reason it’s getting forwarded seems to most often be that someone who got the message felt it would be better addressed on Discourse, so that topic seems most appropriate to be started by the email’s author, not the forwarder.
(But maybe my use cases are actually edge cases…)