The material of a PART can be specified explicitly or it can be specified as any member of some group of similar parts, or both. If a group is specified you are defining a choice of any member of that group. If both a group and a specific material is given, you are again specifying a choice of any member of the group OR the specific material.

If neither the material nor the group is specified (i.e. both fields are blank), the part is referred to as a ‘self’ part. A self part is implicitly referring to itself (i.e. the product or assembly it is within). This is only valid for goods-in, dispatch and dis-assembly methods.