For me a limit is, at minimum, a line I don’t want to touch upon outside of very specific circumstances, and a deep trust in and connection with my partner. I.e. circumstances that won’t happen from simply answering a prompt. With some limits I get instantly turned off and have to walk away. Other limits are closer to “lets paus, undo and not do that again”.

I try to only include limits that I think are relevant to the scope of the prompt at hands. In that sense, limits I list are things that a reader is somewhat likely to associate with the prompt that I really don't want to include or touch upon in a specific RP. Other limits I put in are things I just have gotten a lot of, seemingly regardless of the prompt. Incest, cheating play, racism play and consent play, for example, tends to pop up almost out of the blue in my experience.