Sales tax is applied to food purchases. When a menu says "plus tax" that means the prices *do not* include sales tax. This is the default, if nothing is said, assume tax has not been added. The tax will be computed and added to your bill.
If a menu says "tax included" then the sales tax has been pre-added to the price of each item. In the US this is rarely done, but sometimes vendors will price their items so that with tax they round to whold dollar amounts (or at least no pennies) in order to simplify handling of cash.
Sales tax at Walt Disney World is 6.5%, except at the All Star Resorts where it is 7% (All Star Resorts is in a different county than most of WDW).
So, if you are paying out of pocket at a restaurant, expect to pay a sales tax added to the total.
When you buy a
Disney Dining Plan the sales tax is prepaid. So you won't have to pay any extra tax out of pocket each time you use a credit. This is not so for gratuities (tipping) - you still have to tip, but only at table service.
Also, the price of Disney Dining Plan per day (and the resort packages) *includes tax*. So you don't have to pay extra sales tax to figure out the total cost of your package.
So suppose you buy the regular DDP during high season for $US 46.99 - tax has been pre-paid. This means that the prices on the menus for TS, CS, and snacks need only add up to $US 44.12 since this amount plus 6.5% sales tax equals $US 46.99.