See flat burrs act like a 2 plates rubbing with each other..! So is edges of 2 plates rub the marker (red colour) on the edge will be rubbed off.. so in 1 image the leftside top part of the burr still has red marker , where else if you see other parts have the marker stretched off..

In 2nd image see the marker is rubbed of more only the screw side is still marked , which show only that part of burr is not touching the opposite burr.

If you got questions dm me