A mill test report (MTR) and often also called a certified mill test report, certified material test report, mill test certificate (MTC), inspection certificate, certificate of test, or a host of other names, is a quality assurance document used in the metals industry that certifies a material's chemical and physical properties and states a product made of metal (steel, aluminum, brass or other alloys) complies with an international standards organization (such as ANSI, ASME, etc.) specific standards.
Mill here refers to an industry which manufactures and processes raw materials.