A system is a group of people who live in one body due to childhood trauma. They may have DID, OSDD, or DDNOS. When
the child was young, repeated, and/or severe trauma happened to them. The brain created alters to protect the child.
The host is what you would call
the person who inhabits the body the most, they may be the one who
experienced the childhood trauma.
Remember that DID was once called MPD, but it is considered very outdated to call DID MPD now. Alters are not personalities, they are actual people.