In BDSM, the ‘D’ stands for Dominance. This is the act of being on top meaning physically or psychologically in charge of the submissive or bottom.
It’s extremely important to have a safeword in order to stop a scene when it becomes too intense for either partner. No BDSM play should be conducted without some kind of negotiations before hand, this can be conducted verbally or some people chose to write up a contract, especially if in a Master/slave relationship.
Not all D/s relationships have the same limits. Some aren’t sexual at all, some are all physical or all mental (online) and some relationships develop into 24/7 - which means the dominant partner is control of the submissive 100% of the time.
Limits can be defined in terms of hard, soft, requirement (must) and time and should always be clarified before play. A hard limit is an absolute no-go for either partner. A soft limit is something one person prefers not to have happen, but under certain pre-agreed circumstances are happy to try. A must limit is when an action requires another action. For example ‘If you flog me and pull my hair, I will want cuddles and chocolate as part of my aftercare.’ And lastly time limits. These determine how long an action should go on. So it might be that someone is happy to be caned but only for a couple of minutes.