6-20 mils covers common plastic sheeting, PCB copper foil (1 oz copper = 1.4 mil), and adhesive layers.
Mils are sometimes confused with millimeters: 1 mil is about 25x smaller than 1 mm.
Next stepFor coating work, verify whether your specification is in mils (thousandths of an inch) or microns (micrometers), as mixing up these units is a common source of measurement errors.