When creating a package for an ingredient, you can tick off the "Is piece?" checkbox. To learn how to add a package for an ingredient, read this article.
You would want to tick this checkbox in a few use cases:
When you are selling cans of soda or bottles of wines, one can or bottle is one piece or portion. You would want to tick this checkbox to not worry about the quantity in the can or bottle anymore when you are linking it to your sales item. If the item is sold, the software knows that one piece refers to the set quantity and your inventory will be updated in the right way.
You can buy a lettuce per piece, but when using this in a recipe, you would want to make sure to specify what one piece or one portion of lettuce is if you are not using the whole lettuce.
When using pieces as a unit in a recipe, you want to make sure that the software knows what one piece is. Otherwise, you will not see any costing for this ingredient and if the dish is sold, the inventory is not well updated.