SBing just happens. They are not like tulpas because most of them come from outside the head. For instance, all of mine came from alternate universes and their spirits moved into my headspace. It CAN be done on purpose, so a person can deliberately bond with certain chatacters (a couple of mine were on purpose). SBs also differ from tulpas in that they are usually fictional characters. Like characters from games and movies. Its nothing like DID or schizophrenia, at least according to the two different therapists I've talked to about it.