Quote from Transdude1996
If that's the way it is, then why is there two different categories for the same exact definition?
Split personality is the common term for it, Disassociative Identity Disorder (DID) is the medical or technical term. There are different reasons to promote one term's use over the other on this site; those with a medical/psychiatric/whatever background would say DID is the most accurate term for it, but others may argue that the term is not familiar to many people and would just cause confusion, so it's better to use "split personality" which more people are familiar with. Due to the differing opinions, both terms have been used as tags.
Ideally the mods would agree which would be better and make it a policy, forbidding the other from being used, and if anyone does attempt to use it, they'll be directed to use the one the mods endorse instead.