What is the difference between a “mil” and a “millimeter”?

A “millimeter” is used in reference to the floorings overall thickness, whereas a “mil’ refers to the thickness of the floorings wear layer. Both are units of measurement dealing in distance, the first belonging to the Metric system while the second is from the U.S. Standard system.
(See descriptions below)

Within the vinyl flooring industry, the wear layer thickness is measured in ‘mils’. 1 mil equals 1/1000th of an inch, or 1000 mils would be equivalent to 1 inch. For comparison, common plastic food wrap is roughly 5 mils thick. So, a 6 mil wear layer product is slightly thicker than 1 sheet of plastic food wrap.

When referencing the overall thickness of the plank, it is measured in ‘millimeters’ or ‘mm’. That is why a product can have an overall thickness of 3mm but a wear layer thickness 20 mils.