The Plus box in the Flatness dialog box (Insert | Dimension | Flatness) enables you to enter a tolerance value along the plus direction. This means that any flatness that differs from your nominal or theoretical flatness can still be a valid measurement as long as it falls within the tolerance range specified.
For information on how PC-DMIS reports tolerance zones for form dimensions, see "Reported Tolerance Zones for Form Dimensions" in the "Reporting Measurement Results" chapter.