The various shapes of reference and positioning parts are composed of many surfaces in various combinations, and there are certain size and mutual position requirements between the surfaces.

The datum is the point, line, and surface on which the position of certain points, lines, and surfaces on the part (or component) is determined. That is, the datum is the surface, line, or point on the part itself or related to the part. According to these surfaces, Lines or points to locate other faces, lines, or points on the part.

According to their different functions, benchmarks can be divided into design benchmarks and process benchmarks, process benchmarks can be divided into positioning benchmarks, measurement benchmarks and assembly benchmarks, and positioning benchmarks can be divided into rough benchmarks and fine benchmarks.