While on our trip two weeks ago we used the free Dining Plan at WDW. We used it last year as well. Upon check-in we were given the Dining Plan packet that described the snack option as a "single serve item" such as a drink, whole piece of fruit, ice cream bar and so on. Sounds simple right?
We had 12 snack credits between us to use for the 6 nights we were there. We tried getting a candy bar one time and the CM turned the candy bar over and read the lable. It said it could be divided into 2 servings so it couldn't be counted as a snack (the candy bar cost around $2). I started reading the lables and a lot of the candy or chip and dale snacks can be divided into more then one serving, even the smallest sizes offered. Last year the cut off was $4 with tax. We were able to get cookies, trail mix (as long as it didn't say family size) and other goodies IN THE PARKS last year.
This "serving" rule varied on which location we were at too. The best luck we seemed to have was in our hotel store. We stocked up on the Chip & Dale goodies suck as dried pineapple, almonds, cashews, dried fruit mix and pretzels to bring home. There they still went by the $4 amount and were willing to ring something up to see if it would work. I know the store at the Polynesian even had a whole section set out with things that would ring up as snacks when we went there to eat at Ohana.
While it is defined according to Disney in the Dining pamphlet, it is not always followed in the parks. I think a better definition is needed for guests. This could be anything from a dollar amount set or the bags labled "snack option" or even following how the Poly has theirs set up. It would eliminate confusion and frustration.