This is the question you have to wonder when you come across a science fiction story about time travel : will the author set that story in a multiverse or on a divergent timeline?

           The multiverse : Recently you came across this in Marvel's Spiderman no way home, it is usually always the time travel adaptation used by Marvel, of course you find it in the Doctor Strange franchise but also in Endgame from the Avengers franchise.  In this conception of time travel anyone can access any reality from anywhere. 

          The divergent timeline setting : The best example is Back to the Future where if Marty and Doc being in 1985 REALITY BIS where BIFF is a rich man because of the stolen almanach and they need to get back  to 1955 to prevent old Biff to give the almanach to young Biff (yeah, absolutely impossible you understand this article if you are not into BTTF, but I guess you would not read this article in that case so!) So When Marty and doc are in fake 1985 transformed by Biff they can't get back directly to 2015 to prevent Biff to take the Delorean cuz as doc explains to Marty leaving that actual point in that actual 1985 they will find themselves in a fake version of 2015. This is the reason why according doc they need to get back to 1955 which is that actuall reality 1955 the one where Biff give the almanach to his yound self. 

In this time travel story we come across ONE TIMELINE that gets into different relatities but realities are NOT accessible from anywhere on the timeline, you can only get future, present or past to that same reality you are in. 

I absolutely needed to write this article as for New Vision time travel is a question of multiverse, thanks to the atrium, they do have access to all realities. This is how in 7 parallels they can envision any reality through the smart windows and in 4 enemies you will see that this is why the 7 parallels project is endangered and eveything is to redo...