The June 2012 Tip explained the meaning of a four segment composite position tolerance. This drawing is from that Tip. A composite tolerance has one geometric characteristic symbol but multiple segments.
Datum features referenced in the upper segment of a composite tolerance are there to control the location of the pattern of features. If datum feature references are repeated in lower segments they are there to refine the orientation of the pattern relative to the datum(s) established by the referenced datum features.
A possible gage that could be used to inspect the second segment of this composite position tolerance is shown here. The part must be in contact with the simulators for datum features A and B. Two sliding motions are required in the gage. In other words, the pins must be allowed to "float" relative to the two datum planes established by datum features A and B while remaining parallel to both datum planes. An exploded view of the gage is included so you may see how it might work.