They are both just manifestations of the same thing. DID too. It seems to greatly depend on the medical person's view rather than what the person actually has.

There are hundreds of ways borderline can manifest, but people seem to think of it as what the stereotype portrays. I think people who have problems with impulse control, externalization of anger, self harm, hospitalizations, sui attempts would be more likely to be diagnosed with the borderline label even though they could be considered CPTSD. Other people with the same manifestation of issues would be diagnosed as bipolar (though less likely if self-harm is involved).

Other people, me for example, have not had hospitalizations or sui attempts or a long-term history of externalizing anger. I don't 'self harm' but do self destructive things. My anger was turned back at myself for a long time-i am only just coming out of that recently. And I have attachment issues. I consider these borderline traits, in addition to other things I have (eg dissociation) but I've never had that diagnosis.

I'd be pretty upset if someone gave me the borderline diagnosis as it would be in my chart and could follow me for life. I don't think medical people should use personality disorder diagnoses. Most don't. Discussing it is different than being labeled with it.

If it helps to label and classify your issues, why not see yourself as having both? I have learned about both and it's helped me understand myself a great deal.