they changed the name from 'multiple personality disorder' to 'dissociative identity disorder' because they thought the former suggested that there were distinct persons rather than suggesting that there is one and only one person that is fragmented to a greater or lesser extent. hence talk of 'identities' or ''fragments' or 'personas' instead of talk of 'persons' or 'people' or 'individuals' or 'souls'. some theorists have a grand theory where they cut things up a bit finer... but that is probably the general idea.