While only direct materials and direct labor are traced to a single unit of product, other levels of traceability are also useful in understanding the nature of production costs. The setup costs are directly traceable to a batch, but are indirect to a single unit in the batch; product design costs are directly traceable to the sum of all units of that particular product ever produced, but are indirect to a single units or batch; and so on. This suggests one of the fundamental tenets of cost accounting: different costs are meaningful and useful for different purposes.