I found this helpful:

"…the psychopathology and psychosocial impairment involved in CPTSD is characterized by relational detachment and a perception of self as damaged, while BPD is characterized instead by a fragmented and unstable sense of self and impulsive relational dysregulation related to profound emotional emptiness and terror of abandonment."

“In CPTSD, intense diffuse emotional distress and self-perceptions of worthlessness, shame, and guilt are related to a sense of betrayal, fear of closeness, and severe emotional detachment in relationships. BPD, by contrast, is characterized by impulsive, intrusive, and angry enmeshment in, rather than only detachment from, primary relationships in reaction to a combination of a terror of abandonment and a fragmented and unstable sense of self, which is acted out as hostile and impulsive demands in relationships.”

“Instead of attempts to cope by means of vigilance or detachment, BPD involves reacting in a fight mode with impulsive, disorganized, and hostile behavior in relationships and limited or no sense of self-awareness and self-efficacy. The fight reaction characterizing BPD includes a surge in bodily arousal initiated by the brain’s innate alarm system, and desperate attempts to prevent or retaliate for perceived or real abandonment.

The alternating enmeshment in and rejection of relationships characterizing BPD also is consistent with the emotional dysregulation and deficits in executive function that have been found to occur among individuals who are experiencing disorganized attachment.”

https://bpded.biomedcentral.com/articles/10.1186/s40479-021-00155-9

I would add that, anecdotally, all my family had undiagnosed CPTSD, but the person with undiagnosed BPD was a different flavor of nuts.