With poly bags it refers to the thickness of the bag.
MIL A unit of length equal to one thousandth of an inch.
Then there is .mil part of the .com, .org .gov and .net top level domains used for the military.
Another military use MIL, military unit for defining angles. The name derives from milliradian, and they are used because an angle of X mils is X metres wide at a distance of X kilometres. This means that if you drop a shell 200 metres to one side of the target (according to your map) and it's 4 km away, 200 divided by 4 is 50, so you swing your aim by 50 mils. Mils were first used for military purposes by the German Wehrmacht during World War II.