553. Borer

In an introductory note to this chapter, the Kitzur Shulchan Aruch writes that most Jews are already familiar with the prohibited labors of Shabbos and that he is only addressing common situations with which people may not be familiar. Clearly, the laws of Shabbos constitute an area requiring much greater study than this email alone will allow. 80:15 If food is mixed together with something undesirable, one may draw the food from the undesirable thing, but not the undesirable thing from the food. Also, the food may not be taken with a utensil, only by hand, and one may only take what he needs for immediate use. Even when the mixture is all types of food, if he wants to select something to eat immediately, he must be sure to take what he wants for now and not what will be put aside for later. This is because what he wants to eat now is considered "food" and what he won't use until later is considered ''undesirable." One may not even peel garlic or onions and put them aside for later because of this melacha, which is called "borer" (selecting). Rather, one may only peel that which he needs for immediate use. One may not remove the outer peel that covers  all the cloves of a head of garlic even for immediate use. This is considered mefarek (separating), which is a subordinate activity of dosh (threshing).

80:16 Non-food items like utensils are included in the melacha of borer.  Those required for immediate use are like "food" and the other items are considered "undesirable."