The Minus box in the Angle dialog box (Insert | Dimension | Angle) enables you to enter a tolerance value along the minus direction. This means that any profile that measures less than your nominal or theoretical profile can still be a valid measurement as long as it falls within the tolerance range specified.
PC-DMIS allows positive-lower tolerances (or a positive tolerance in the negative range). This is done by typing a minus sign before the value in the Minus box.