A common datum refers to a datum identifier like A-B or A-AB-B. It has one or more hyphens that separate defined datum identifiers. PC-DMIS supports two types of common datums:
Coaxial cylindrical surface. These may have different nominal sizes and/or size tolerances. You may measure them as either cylinder features or circle features.
Offset parallel planes
For more information, see "Common Datums:_Coaxial Cylinders" and "Common Datums:_Offset Parallel Planes".