I was confused over the alcohol vs. sugar alcohol as well (still not totally clear), but it looks like sugar alcohol should not be entered into FitDay. It is already accounted for in carbohydrates. I ran across an interesting article that indicates sugar alcohol is neither sugar nor alcohol, but is so named because the chemical properties are similar. (What are Sugar Alcohols?
My only guess for the "alcohol" field is so you can track how much alcohol you actually consume...perhaps if you have 100 grams of champagne that is 10% by volume (numbers made up for simplicity), you could enter 10g in the alcohol section. It seems like you would still need to account for the calories, however.