Without the modifier specified for Datum B the Datum is established at RFS.. With a CMM, one would establish several circle starting at the top of the as-built hole features as well as several locations spaced along the length of each Datum B hole feature. From these circle measurements the CMM would establish a Datum axis.
To establish Datum B at RFS in a fixture -- one would design the fixture with two expanding precision dowel pins or similar feature located at the basic dimension location and/or orientation. The as-built part is placed in the fixture with the expanding dowel pins collapsed and then the Datums are established and maintained in the order of precedence specified.
Expanding Pin Example