The Minus box in the Surface Profile dialog box (Insert | Dimension | Profile | Surface) 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.