My T and I recently both signed a contract, as well as many different alters that I have. One part was for her to sign... that she would not abruptly end treatment unless something beyond her control forced her to do so. We had to sign that no one in the system who has been involved in therapy would abruptly end therapy either, without a closure session and a chance for our T to explain why it's in our best interest to continue. This was draw up by all of the alters and our T, to help with abandonment fears.
There is also a section on self harm behavior. It doesn't say that we won't self harm, but that we will try to contact a safe person (our roommate, our T, etc) if the urges to self harm get too strong so that they have a chance to help us be rational. With alters sharing my body, it's just trying to plant in a few speed bumps for those alters who tend to crash out of control.
Alters sign if they wish. If they don't sign, that's their choice. It's mostly a security for us that we won't be abandoned, and that we will try to reach out if we want to harm.