Hi,
If you are eating at CRT, HDDR, Aloha Dinner at Poly, and Mickey's BYBBQ your tip is included even when on the dining plan. (you do not need to add a tip)
If there are 6 or more in your dining party, your bill at any TS restaurant (including buffets) will have an 18% gratuity added. While the dining plan covers the cost of your meal, it does not cover your tip. You may pay for this 18% additional charge in any way you desire. (cash, cc, debit card, Disney gift card, etc)
If there are less than 6 dining, you may tip any amount at your discretion. The bill will be presented to you with the amount of your meal. You are not responsible to pay for your meal (
DDP covers this) but your tip is not included. You may again add in the tip with the payment type of your choice. This is not an automatic gratuity charge if you have 5 or fewer people in your party. (BTW infants count)
Double check your bill, very few places at WDW include the tip.
Hope this clarifies it!