The box item type is helpful to use in scenarios where you buy an item by the case but you sell it by the case, in smaller packs or as a single. An example would be a case of 24 sodas that you might sell in packs of 12, 6 or individual cans as well. By creating a box to link the case to its packs and singles, their shared inventory is automatically broken down or built up when you checkout your customers. You can also manage your inventory manually beforehand if you prefer.
NOTE: A box needs to be made up of the same item. If a box is intended to be made up of different items, like a custom bicycle or gift basket for example, then this would be an assembly.