There are different things being discussed here, and about 5 basic parts of most chickens' diets. I think maybe the OP is not using the same wording for parts of chicken diets as some of the rest of us are. These are generalizations, and I know that some people do things differently, but as a general rule:
1. Good quality chicken feed (pellets/mash) should generally be the bulk of all chickens' diets. Commercial feeds are developed with nutritional needs of poultry in mind, and the right balance of protein and vitamins.
2. Oyster shell. This is ground up oyster shell that laying hens should be given as free choice to keep up their calcium levels for healthy shells and bones. Some people refeed egg shells to their chix instead of or in addition to oyster shell. I just keep a jarful wired to the cage side near the food. Oyster shell is NOT a substitute for grit. Oyster shell is broken down and absorbed in the guts and used to lay down eggshell.
3. Grit. This is essentially small stones/ground up granite. All chickens should have access to grit once they eat anything except chick/layer feed, including grass, scratch, bugs, veggie scraps... anything. They swallow this and it goes to the gizzard to help grind of food; there is no nutritional value to grit itself. If your chix have access to soil and small stones, you may choose not to buy grit, but without adequate small stones, your chickens' guts will not be happy, and they won't be able to digest the other foods here.
4.Treats: veggie scraps, fruit scraps, even meat scraps can be a treat for your flock. Some people give sunflower seeds, or oatmeal. Bugs, mice, grass, and weeds fit into this category as well. You should steer away from scraps with added/refined sugars and salts. Most things you'd put in your compost pile are OK in moderation for chickens.
5. Scratch is a store-bought or homemade mix of various grains for your chix. Most poultry-keepers think should be treated like "candy" and given in moderation. Various store carry various mixes of scratch in different seasons. Dried corn, in particular, as a part of a "scratch" helps keep body temperature up, which can make it a good wintertime treat when the girls are using lots of calories just staying warm.