You can order anything (entree, dessert, non alcoholic drink). If you decide to order an appetizer or alcoholic drink, you would just pay the menu price for those items.
The entitlements remain the same so you can choose any entree, dessert and fountain drink at a Signature when you are on the dining plan. There are no entrees that are excluded.
The only Signature that might have a surcharge is at Monsieur Paul if you order the prix fixe dinner when using regular dining plan credits.