Before the change in rules, this is how it used to work for me, who never carries a balance.
Disney vacation would show on a separate line at 0% and it's balance. However, the statement balance included this amount.
Now, to make matters worse, is that if after paying your bill in full, any credits you might have afterwards that were more than any new charges (ie you return something to the store), would be applied towards the disney vacation balance.
Therefore, it was important to take a close look at that special balance information which would more likely be less than the original charge, and only deduct that amount.
For example. On a particular month, you charge $1000 dollars, and a $200 disney vacation. Statement would show a balance of $1200, but on the finance section, you would see a 0% promotional offer balance of $200.
You then pay in full the balance, except for the disney deposit, ie $1000.
Next you return a purchase of $50, before you make more charges. Next month comes around and your total statement is $1200. However, that $50 credit was applied towards your special interest, therefore that balance is now $150.
If you make a payment of $1000, you would be faced with interest charges of $50 on your next statement. Instead you needed to pay $1050 to maintain that special balance.
You are probably LOST by now, but it is how it used to work. I doubt it'll be much different with the new credit rules. Won't know until I pay by balance in a couple of months...