This is the stock remain value on the surfaces. E.g. if this value is set to 0.2mm, then the tool will not come closer to the surfaces more than 0.2. Accordingly after the machining, there will be a stock remaining on the surfaces about 0.2 mm. Note that the negative value cannot be bigger than the tool diameter. In case larger negative values required, consider using the "Axial Shift" in the "Utility" page.