It does not mean that buyetr cannot supply any materials. Sometimes it has to, however, the bottom line is where this "substantial part" lies. There are three ways to define: economic value, vilume, respective contribution to the end product. Most scholars favour economic value test. This means that CISG does not apply where the value ofb contributions of a buyer to the supply of materials is 50% or more. The importanrt rule is to compare party's contributions to each other and not to the value pf end product.