Among the options to "leave" i.e. logout or shutdown my desktop, there is also the option to "start a parallel session as a different user". Can someone explain what this means, and how it is different to simply logging out/in? Googling turned up lots of information about concurrent ssh sessions, but I don't think that's the same either.
I had an issue recently, which I think was related to parallel sessions, when I logged in after another user had been using the computer. I think what happened is that I left my session running, and the other user started a parallel session, rather than logging me out. When I logged back in, there was already a firefox process running under my UID, preventing me from starting a new firefox process.
21 Answer
Ubuntu supports multiple user sessions at the same time. That means you can log in user A, then lock the screen and/or switch users and log in as user B in parallel.
Both sessions are open this way and no applications will be closed. You can switch between the open user sessions at any time or open new sessions as other users. Each session stays open until you log out from it.
User A and user B have completely separate sessions which run independently of each other. It is not really possible that another user e.g. starts applications in your session, as you describe, unless maybe if the other user is an administrator and explicitly launched the process under your account.
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