I checked with my psychiatrist friend about the question of whether self harm is considered features of both, or more indicative of BPD traits. She said:

"It would depend for me what was driving the self harming. Often cutting is seen as synonymous with borderline PD, however self harming can occur in many disorders.

For me if they are self harming to relieve overwhelming emotions, when under distress, to punish themselves or in reaction to abandonment fears, I generally equate that more so with borderline traits.

You would have to contextualize the symptoms with the patient's history.

Sometimes the two disorders overlap, especially in cases where a patient may have a mother with bipolar who is frequently in hospital which may be perceived as invalidation, leading to borderline character while at the same time having the genetic predisposition for bipolar.

Hope that helps?"

So that's one explanation. Gives a little bit of clarity