A datum pattern refers to a datum identifier like A or AC that refers to multiple similar features of size that all have the same size tolerance. For example, a pattern of holes might be referenced as a datum pattern B. Technically (per the ASME Y14.5 and ISO 5459 standards), a datum pattern is a type of common datum, but this documentation does not use that terminology.
For more information, see "Datum Patterns" in "How PC-DMIS Solves and Uses Datums".