Many production processes result in more than one stock keeping unit (SKU). These additional materials may be Co-products or By-products and SyteLine models both.
A By-product is something that is a residual of or incidental to the production process. You are not in the business of making the by-product, and if you could eliminate it from the process, you would. But since it has value, you need to inventory it. Co-Products arise when you create many SKUs with the same job.