Figure 2. Planner datums and simulated datum plane

A datum feature is a part feature (or FOS), that contacts a datum during measurement. A datum is a theoretically exact plane (or axis or center-plane), from which dimensional measurement should be made. However, in actual practice the theoretically perfect plane is simulated by a gage surface, which is best possible theoretically exact plane, such as surface plate. Such gage surfaces are called datum simulator.

Figure 2 explains this concept. Notice that both the datum feature and the simulated datum are not perfect. During measurement, the datum feature will contact the simulated datum which in this case is a surface plate. The measurements that refer to this datum will be made from the surface plate.