A dial indicator mounted on the end of the drill bit is used to measure the plate. The mounting plate is required to be equal on all sides, front to back and side to side. A properly leveled plate ensures the cylinder head is cut correctly. A CNC machine is nothing more than a computer-controlled machine that follows a pattern that the operator designs. If the cylinder head is not mounted right it could get damaged. That leads to expensive repairs or worse, a ruined cylinder head that gets sold as scrap.