The clean cut thickness refers to the thickest the material you’re cutting can be for the machine to cut it all the way through with minimal dross on the bottom edge that needs to be cleaned off when the cut is finished. The severance cut thickness is the maximum thickness the material can be for a close cut at a much slower speed. A sever cut will also require more cleanup on the bottom edge as it won’t be as clean as the clean cut. Going beyond these thicknesses will alter the results and the speed of the cut, making it more difficult to get the cut you want.