So there are hard limits and soft limits in BDSM. A soft limit is a practice where either the submissive or the dominant (Yes, dominants have limits too!!) is willing to attempt but is unsure. The uncertainty might come from having never done it before, the practice being a trigger, health reasons, or fear. Nonetheless, soft limits need to be WELL COMMUNICATED.
A hard limit is a BDSM practice where the submissive or dominant WILL NOT AND DOES NOT want to attempt! RESPECT these limits. Most people have very good reasons for hard limits. Don’t question the validity of the limit, just accept and respect.
For example, I have a hard limit against breath play. As an asthmatic I simply cannot have my breathing constricted in any way. I’m also mildly clausterphobic so being encased in a very small area is enough to send me into a panic attack.
A soft limit for me would be bondage suspension. I’m fascinated by it but have never tried it. The idea of it is intriguing, but because of the clausterphobia, I’m skeptical.
As will all things, communication and respect are key.