Google Snack Pack box is the top 3 map business listings box, that appear in some of the results for organic searches in Google.
Unlike the organic results (and ads), the Google snack pack box appears somewhere in the first page, among other organic results, and has also an inner order of the local businesses.
Rankings are represented by a number (The rank of the box) and a letter (A-C - the position of the business within the box).
For example, if there are two organic results before the box (the box's rank is #3) and the business that PRT tracks is in the second position within that box (B) the rank displayed will be 3/B.
To clarify, for the screenshot under: "Hub Plumbing &..." rank is 1A, "Bargain Plumbing..." rank is 1B, "Franco Belli..." rank is 1C.