A CNC milling machine requires specialized software that manipulates the cutter, moving it not only front to back and left to right, but up and down.
Lasers capable of doing relief carving also require specialized software; however, while it moves the lens in front-to-back and left-to-right aspects, it does not move it up or down. The lens stays focused on the surface of the material. Depth is achieved by varying power based on grayscale color values. The darker the grayscale color, the more power it used and the greater the depth of the carving. It does not do this one-color value at a time, but continuously line by line.
-Jim Puentes, COOLaserCraft