Dissociation is a continuum with everyone dissociating sometimes; when reading a book and getting "into" it so you're unaware of what's going on around you or driving somewhere and arriving without remembering the trip, etc. Those are at the mild end of the line with what use to be called MPD (but is not Dissociative Identity Disorder) pretty much at the other end.

For my purposes I think of a dissociative disorder as a person not having control over one's attention and it becomes a "disorder" when it interferes with one's life. If one has additional identities, has one's attention/life "split" in that way, that would pretty much make one's life difficult I'd think!