It's not to do with the level of violence that's for sure. As with many subs, face slapping (with hand or cane or something...) is a hard limit and something I'd find extremely humiliating, even unacceptable. Close to the same for being hosed with water or food in the face - but on the other hand the thought of being on my knees giving head, deep throated and hair pulled is a strong turn-on and something I hope to bring to r/l soon. I don't think I'd refuse being cream pied in my face and told "you need this, hoe" in that situation. It's very much to do with if it feels close or just distant and cold.