A datum feature simulator is the manufacturing or inspection equipment contacting the datum feature of the part. Datum feature simulators are used to establish the simulated datums.
A simulated datum is a point, axis or plane established from the actual surface of the datum feature simulator.
ASME Y14.5M Definitions
Datum feature simulator: A surface of adequately precise form (such as a surface plate, a gage surface, or a mandrel) contacting the datum feature(s) and used to establish the simulated datum(s).
Simulated datum: A point, axis, or plane established by processing or inspection equipment, such as the following simulators: a surface plate, a gage surface, or a mandrel.