You probably won’t find an academic source about it because it’s not academic! I’m not sure about the history of the term, but I do know it was borne casually from the bpd community. Other personality disorders have coined similar terms, like “safe person” for avpd.
It’s meant to describe someone for whom your bpd symptoms basically fixate on them. Splitting between love and hate, idealization and devaluation, jealousy and self-degredation. It’s someone you want to be in contact with 24/7, love with all your heart, think the world of, think they’re perfect, feel like you would die without them, would do anything for. Emotional dysregulation, self-destructiveness, and abandonment issues are at peak height with this person. A bit of attention and validation give you a great high; a bit of inattention gives you a great low. The difference between having an fp and not can seem so jarring that it can feel like you have bpd with an fp and don’t actually have the disorder after all without an fp.
So people use this term to describe someone they are especially symptomatic on and want companionship and validation from, more than anyone else. But this isn’t usually someone who consents to being their source of validation. It’s whoever the person with bpd becomes obsessed with. Therein lies the issues I described.
Plus you can have multiple fps which like. Defeats the point of the term lol.