The term "resident" is used to describe everyone inside of the system, even if it's temporary, and regardless of where they came from. This catchall term is used to avoid terminology that comes from the psychiatric community, so that multiples who have unpleasant or torturous experiences in treatment can feel more comfortable, and because the term "resident" helps externally sourced persons in the system feel included, the term contains no insult of believing the people are less real or respectible than people who have one-body-per-person (cf. "singletons").
This term borrows from the possibility or metaphor of the internal landscape being some type of domicile or physical area, such as a house, hotel, world, or country, which is exceptionally common amongst multiples. The persons living in a house, hotel, world or country at any given time are it's residents.