It's all in how you use it. The point is to document a process. This can show how an intent is expressed. If two parties conform then intent is expressed as expected. Otherwise someone gets surprised. Your question centers around who's problem that is. Either change must be made to conform or thisthe expression, the standard (yet another version), or we abandon usinglet the standard as littlebecome nothing more than a suggestion.
It is certainly decriptive. Wether it's proscriptive depends entirely on who your working with. If it's not in the contract it's just something to read when trying to get to sleep.