What the Image in Depth Tab Under Brush Represents

From this view we can see that the belt is quite thick

and it is embedded into the skin as well as extrude out of the surface of the skin.

I want the back edge of the belt to butt against the skin surface as close as possible.

In order to achieve that, go under Brush then Depth tab.

There is an image with a black dot and a white circle with a line running across the center.

The top of the circle is the highest point of the strap,

the bottom of the circle, the lowest.

The black dot, the center position. The black line the surface of the skin.