What is the difference between a co-product and a by-product in SAP?

Certain manufacturing companies produce additional products related to their other products which have some financial value. These products are either co-products or by-products. Co-products can be sold independently or used in other manufacturing process while by by-products have significantly reduced financial value. Both co-product and by-product are characterized by having negative quantity in the BOM;  however, it is relatively easier to setup the master data of a by-product than of a co-product.