I am still learning about the dining plan and its features, but I have assumed a snack was:
Your Snack includes your choice of one (1) of the following:
You can find Snack locations throughout the Theme Parks or at Counter Service locations in the Resorts. Most cart locations selling frozen ice-cream novelties, popcorn or Coca-Cola products accept your
Disney Dining Plan.
Frozen Ice-Cream Novelty, Popsicle or Fruit Bar
Popcorn Scoop (single serving box)
Single Piece of Whole Fruit
Single serving Grab Bag of Frito-Lays Chips
20 oz. Bottle of Coke, Diet Coke, Sprite or 24 oz. Dasani Water
Medium Fountain Soft Drink or Apple Juice
as cut and pasted from the Dining Plan brochure.
However...

it could always be at the discretion of the establishment employee I suppose.