@fengjingchao 
What I am doing by this field is to pick out the most recent Accept, and I think there may be some preAccept replies that have larger ballot than any Accept replies. Let's say the max ballot among all Accepts is 1, and the max ballot among all preAccepts is 2. Then if I don't have this field, I cannot pick out the most recent Accept when the preAccept comes first.
Do you have any other idea?