If you check the whole box, it indicates that you can only purchase whole quantities of the item. For example, packs are usually whole (you cannot purchase a fraction of a pack), items that you purchase by weight are not whole.
Checking the whole field will change how the item is added to the shopping list, as the quantity of the item will be rounded.
Fruits and vegetables are often sold by weight - and not by the number of pieces, as the weight of each piece may vary. If you can pick up the number of fruits or vegetables that you want and weigh them in the shop to determine their price, then you should not check "Whole". Although you cannot purchase a fraction of a fruit or vegetable, you can get close to the quantity you need by selecting pieces of different size for example.
Same thing for the meat that you buy at the butcher's.
On the other hand, if the meat, the fruits or the vegetables are in packs with a fixed weight, then the "Selling unit" field should be set to pack, the "Size" field should be the weight of the packs and you should check "Whole" as you cannot purchase a fraction of a pack.