Insert a round, rectangular or oblong sink
This article is part of a TARGET 3001! Crash Course Frontpanel.
After clicking the icon 
the following dialog appears:
Image 1: The dialog for placement of a flat milling (sink)
With a flat milling you do not have a break through the material. The tool penetrates only partly into the material so that a "material background" remains. With a flat milling you can not penetrate deeper than the thickness of the material itself. Ticking the box "rear" gives advice to your manufacturer that the flat milling shall be applied from the backside i. e. to press threaded bolts or bushings into it.
Image 2: A 40 mil (1.02 mm) flat milling