Everybody has "hard" limits--things that they absolutely will not do, and will not even consider. Some people, for example, like to be tied up but don't like the idea of being whipped; if they won't allow themselves to be whipped, ever, that's a hard limit.

There are also "soft" limits--things that someone won't do under ordinary circumstances, but will allow to be "forced" on him or her in the context of a particular scenario that's being acted out. Between soft limits and hard limits lies an interesting psychological territory to explore.