Negotiation and limits
Negotiation is the conversation before. What is in, what is out, what the word is that stops everything, what each of them wants and is afraid of. In practice it is the part of BDSM that practitioners consider non-negotiable, and in fiction it is the part most often cut, on the theory that it is not sexy. Readers who tick this theme disagree. The conversation is where two people say what they want out loud, and there are few things on a page more charged than that.
Limits are the negotiation's product. A limit the reader knows about is what makes a later scene tense, because the reader knows where the edge is and can feel the story approaching it. A story without stated limits has no edges, and a scene with no edges has no tension.
How the engine handles it. With this theme ticked, the negotiation is written as a scene rather than assumed: the terms are established between the characters before they are tested, either in dialogue or through a history the prose makes clear, and the story state carries them so that the terms hold in every later section. A limit crossed is a story event, not an oversight. Separately from the characters' negotiation, you have your own: the special instructions field is where you write what the story may and may not do, in sentences, and those travel with the story into every section. Your limits and the characters' limits are two different things, and the one limit that belongs to neither, every character in a sexual context is an adult, is a third. The BDSM guide keeps the three apart in detail.
Your stories are encrypted at rest. We can read them to write them. Nobody else can read them.