honestly this is just a term i coined(?) because it's my way of describing my own experience!
it's really complicated for me, im not quite sure what exactly it is i experience but it's likely tied to several illnesses of mine; to make a VERY long story short, they're in no way "individual" enough to be their own alters - i have osdd-1b - but still separate enough to be identified as "someone"(???) else. i understand this probably is really vague and confusing, but i don't understand my own experience well enough myself. just because im diagnosed with these disorders doesn't make me an expert. similarly enough, i have a record in my life of "splitting" different abusers up into identifiable "fragments". the best example i can give was one of my abusive exes who i split up into different perceived parts/names that represented the "goods", the "bads", and the in betweens. some parts were more distinct than others. it allowed me to shift blame or "identify who was mad at me". it was a subconscious thing and took me an extremely long time to realise it was not normal.
most likely it is a combination of trauma responses and such from a bunch of different disorders. i split myself up as well because of my identity diffusion in bpd and, again, the sense that it is almost someone else thinking and doing these things, which could be from both bpd and osdd. i wish i could give a more distinct/clear answer, sorry!!