This article gives an explanation about what a short sale addendum is that might help answer your question. If you have any doubts, you might consider speaking with a real estate lawyer and having him/her look over the document.
It was a question about a bank addendum, not a short sale addendum. The problem is our real estate agent didn't disclose new conditions(like we/buyer have to pay for sellers unpaid taxes) when short sale was approved (before going to escrow). We found out about it only a day before closing.