Generally, mil is used to describe the gauge of linear low density film. A micron is usually used to describe the thickness (gauge) of high density film. The higher the numbers, the stronger/thicker the film.
Examples:
1) “We currently use a 40×48, 1.5 mil can liner. Is this the right bag based on our operations?”
2) “The 10 micron bag we’re using is not strong enough for our application. Please send us a sample of a 12 and 14 micron liner.”