I would have thought that any relationship you can back out of at any time, contract or not. What may be useful in such a contract may be an 'exit strategy'. i.e. rather than merely saying 'yes you can quit any time' but also adding how that works - what needs to be done to make the end of the relationship 'official', what grounds you can use for quitting (i.e. 'he broke my hard limit more than twice when I told him not to'), who gets to keep the precious collection of signed floggers and so on. Sort of like a pre-nup in some ways. I would also include things like:

- is the relationship polyamourous? Can the Dom or sub play around with others? Just 'BDSM play' or is there sex as well? Can the Dom 'give' the sub to another for a session and can the sub disagree?

- A list of punishments and what crimes they are for.
- What duties the sub has and also what responsibilities the Dom has.

There are many other things that could be added...