As you begin to spend a bit of time in the lifestyle you'll hear of documents referred to as "slave contracts".  These documents are also known by a multitude of other names so don't let the nomenclature fool you.  They all serve basically the same intent.

 A slave contract or other written agreement is not required within a D/s  relationship, regardless of whether you are submissive or slave.  This  type of agreement can however be used to clarify boundaries, limits or  intents and may be worth considering.  

 It's worth noting that a "slave contract" is not legally enforceable in the United States nor is it in many other countries.  While there are many that will argue differently, it is my opinion, and the opinion of some  attorneys that I've spoken with there are some documents that can  be written that are enforceable if they are written as stand alone documents some things that can be written up as separate and unique  documents that may be enforceable (such as powers of attorney, labor  or services agreements, rental agreements, etc.) so always use care  in signing anything.  If you don't intend to live up to the terms of the  agreement, you shouldn't consider signing it!