My official diagnoses are DID, PTSD, MDD and anorexia nervosa. My PTSD is more like C-PTSD but in my case I have been told the DID diagnosis supersedes the C-PTSD diagnosis (as in no point being diagnosed with both of them) so I have the PTSD diagnosis to cover those symptoms that fall under that umbrella. In my case the DID supersedes the C-PTSD for those symptoms that are not included in the C-PTSD umbrella (two or more distinct identity states and amnesia).
It isn't an exact science and different people in different countries with different training and different perspectives may or may not call the same things the same thing (or call different things different things).
What seems to matter most is having a diagnosis that enables you to access the specific help you need.
Edited to add I don't fit the criteria for a diagnosis of Borderline PD or any other PD.