your basic rule of thumb for what's a snack on the dining plan is
1) The item is priced under $4.00 AND
2) it is a single-serving sized item
so, at the resort convenience store a 6 pack of doughnuts for $3.99 will NOT be allowed as a snack, because even though it's under $4 a 1/2 dozen doughnuts is not meant to be a single serving. Likewise the 1/2 gallon of milk priced under $4 - not a single serving.
But, a giant gourmet double chocolate muffin priced as $2.49 IS allowed because it meets the criteria.
There are a few exceptions, here & there, at some food carts. Some menus (and food racks in food courts) will have the DP emblem on them to let you know they're on the dining plan. But if all else fails, ask the counter person