How does Ue4 find who’s joined data to load?

I’m working on multiplayer saving and I got to wondering how does ue4 deter an who joined the server
So if one player was on the server/client1 and they left if another player joins would they load up client1s info or would it just create a client 2.

It looks like I found the wander to my question what I’m doing is using this plug-in Advanced Sessions Plugin - Community Content, Tools and Tutorials - Unreal Engine Forums then I can use their node to get my player steam ID