For example, imagine you are linking a french fry recipe to menu levels 1, 2, and 3, where 1 is a small size, 2 is a medium size, and 3 is a large size. A small is one portion of the recipe, a medium is 1 1/2 portions, and a large is 2 portions. Therefore, you would enter 1.5 in the quantity field for menu level 2, and 2 in the quantity field for level 3.
Parent topic: Menu Item Linking