See also environment-dependent states.
Memory has been noted to be an intricate web where memories are grouped by associations and linked to one another very much like webpage links.
One does not have access to all of one's memories at any given moment. Some memories are closer to the surface than others. Some are immediately accessible, some are readily accessible given a moment, and some will take a lot longer to call forth if one tries.
There's a few mechanisms that determine which knowledge and memories are closest to the surface at any given moment. One is called "priming" in psychology -- something in the environment has called forth a bunch of associated knowledge and memories, and now that information is closer to the surface than it might otherwise have been. Priming can last for minutes, hours, days, etc.
Thus whenever one changes environments and thus states (see that there page, environment-dependent states), one also changes the repertoire of available knowledge and memories. This is normal for singletons.
To take the concept one step further for multiples, who one is at any given moment is, in some ways, an exceptionally complicated state. Each system has associated memories and memory groupings of things that happened while in that "state" i.e. while that person was front. It is thus easier to recall what happened when Joe of Joe& was front last week if Joe of Joe& is front now. If Sue of Joe& is front, she will have to poke at Joe's memories (if there is shared access) or ask him internally to recall what happened when he was fronting.
(I will eventually tie this all together better, but I'm not up to writing a thesis paper right now. honestly, there are places to go with this and the other section -- Crisses)