Scratch is a mix of cracked grains. In the past it was a way to use surplus or spoiled grains tha might otherwise be unusable.
When purchased commercially, it usually consists of wheat, corn, oats, sunflower seeds, millet and various other seeds.
It is NOT complete nutrition. It keeps them busy and feeds their need for such energy foods as found in grains. It is ground coarse, to a size preferred by chickens.
For complete nutrition you must feed an all around poultry ration, either one you make yourself or purchase commercially. The commercial feeds have wheat, corn, soya, fish or protein meal, vitamain/mineral fortifiers, binders and really do offer a complete meal.