Simple definition Erotic pain is anything that turns the reciever of the pain on. I know Mishka well enough to know that almost anything I did to her would be erotic, but if someone else did the exact same thing, in the exact same way, it would be non erotic.
The reason some people have trouble with the concept is that pain is not supposed to be fun. We learn early to avoid pain because it hurts. Yet our body actually responds to pian by producing endorphins that cause pleasure. Then we get into the mind set. I could actuall cause physical damage with the intent to hurt someone, and that person would find it erotic.
trying to define erotic vs non erotic by the intentions of the person causing the pain, or by the type of pain, is useless. this is an entirly subjective definition that resides in the mind of the person in pain.
A few years ago there was a movie that came out of Hollywood. (Crash 1996) It dealt with a fringe element in the BDSM world. Erotica is, and always will be, in the mind of the beholder.