I will let Al answer this question for you Wade. I can say that some states allow the addition of sugar, realizing that in some years there is not adequate natural sugars. There are also rules about the addition of water, even when used to dilute the acid. You are allowed to add about 10% of the juice as water, but it can vary a bit.
Al can confirm this, but I think the alcohol level is 14% or lower for the normal tax rate. Above that is taxed at a higher rate.
Sweetening is allowed also as far as I know, at least here it is. Some wineries overdo it though in my opinion, but they are making it the way a lot of people want it- oversweet!