too general a question, i recall a chat with a friend who is a psychiatrist on p/d.s his answer- labels made by men in suits! they are observed behaviours which do cluster in people. i have met two people with bpd who i found extremely hard to figure out ,i'm generally good at it .but i used to find i'd get too close to them and then it was ---back off, they ended the friendship not me. i found their energy to be intoxicating, but their sensitivity- ouch! treading on eggshells is an apt description, i still wish they were my friends