What is a "self-map"?
A self-map is usually an attempt by a system to illustrate the interrelationships in their head, or to give a map of their headspace. Sometimes it looks like a map of a landscape, a house or building, sometimes it comes out more like a flowchart or family tree. Or anywhere in between. Essentially it's a diagram of some type attempting to translate social and working relationships between people.
Mapping your system may be associated with being in therapy or an effort to explain your group to another person who wants to understand details about your system. It is by no means necessary to map your system simply because you're multiple, however it is a possible tool that you can use if you desire to try it out. Therapists specializing in DID/MPD have historically encouraged mapping because they felt that the map made it easier to identify persons whom they believe embodied defenses and fears that could be worked on in therapy, or that maps might inherently show areas where other alters might be hiding. One type of map encouraged by some therapists was explicitly to pair opposing alters, to somehow prove that this juxtaposition, if integrated, would create a balanced centered person.
Reasons to create headmaps for oneself include locating missing links or people, allowing people to identify themselves and their connections, mapping out communication channels so that you can perhaps improve on them, and helping other people in the system who are not fully co-aware get a better idea of who else is living in their head with them. Maps can help internal people find their way around in an internal landscape, or figure out who to talk to internally about certain things, and help develop interrelationships with people internally. They don't need to be shown to anyone else, even though that's another viable reason for wanting one, nor do they need to be drawn in any permanent/written form at all.
A really nice article about different ways to map ones system. (at the Internet Archive)