MIL ~ The unit used in measuring the diameter of fiber strands, wire, and so forth. 1 mil is the distance equal to 0.001 inch: a “milli-inch,” in other words. Mils are used, primarily in the U.S., to express small distances and tolerances in engineering work. One mil is exactly 25.4 microns, just as one inch is exactly 25.4 millimeters.
mm ~ millimeter: a very common metric unit of distance. One millimeter equals 0.001 meter, 0.1 centimeter, about 0.039370 inch, or 39.370 mils.