I see in your case it is probably about vinyl.

Clear vinyl is a durable material that comes in many thicknesses. With clear vinyl, the higher the gauge, the thicker it is. 60 gauge is thick at 0.06 inches and 4 gauge is only 0.004 inches.

https://www.onlinefabricstore.net/makersmill/clear-vinyl-fab...

How do I know it is probably about vinyl?

https://supercomfysleep.com/most-durable-air-mattresses/

https://www.bestitemes.com/best-camping-bed/

Understanding Vinyl Pool Liner Thickness

When talking about the “mil” or “guage” of vinyl pool liners, you should first get a decent understanding of what these terms actually mean. The most common term for vinyl, “mil” (not to be confused with millimeter) is a unit of measurement equal to one thousandth of an inch. Therefore, a 20 mil liner thickness is equal to 20 thousandths of an inch.

The term “gauge” (not to be confused with mil), is a bit more of a moving target. See, multiple industries measure items such as wire, shotgun barrels, sheet metal, photographic film and hypodermic needles by their gauge. This can be very confusing as in a case like shotgun barrels where the smaller the gauge, the larger the barrel. This is not the case when it comes to vinyl pool liners where the higher the gauge, the thicker the material.
When shopping for pool liners, it is very important that you are aware that gauge is not equal to mil. Some manufactures or retailers like to offer their liners in terms of gauges because they sound thicker than a true mil.

To be technical, a 30 mil liner is equivalent to 0.0300 inches.
A 30 gauge liner is equivalent to 0.0260 inches (26 mil).
Therefore, if you see a liner advertised at 35 gauge, it is truly a 30 mil liner. To avoid this confusion, always make sure and purchase from a company that advertises in mil, versus gauge.

https://www.secardpools.com/pool-school/understanding-vinyl-...

Volgens de eerste ref is een gauge = 1/1000 inch (dat gaat dus eigenlijk over de 'mil')

Volgens de derde ref is een gauge < 1/1000 inch

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Note added at 1 hr (2020-03-12 16:00:30 GMT)
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In feite eens met Willem.

Het gauge-verhaal is iets ingewikkelder of breder dan ik dacht.

Meestal (?): hoe groter de gauge hoe kleiner de dikte /diameter.
Bij vinyl /PVC niet.

Voor naalden:

The gauge starts at the lowest gauge number of 5Ø or 00000, corresponding to the largest size of 0.500 inches (12.7 mm) to the highest gauge number of 36, corresponding to the smallest size of 0.004 inches (0.10 mm). Size steps between gauges range from 0.001 inches (0.025 mm) between high gauge numbers to 0.046 inches (1.2 mm) between the two lowest gauge numbers and do not correspond to a particular mathematical pattern, although for the most part the steps get smaller with increasing gauge number.[3] Concerning wire and fine tubing, the gauge number is used to specify the outside diameter of the product, whereas for larger mechanical tubing the gauge number specifies the wall thickness independent of the overall size of the tube.
In medicine, the Birmingham gauge specifies the outside diameter of hypodermic needles, catheters, cannulae and suture wires. It was originally developed in early 19th-century England for use in wire manufacture, and it began appearing in a medical setting in the early 20th century.

Hypodermic needles are available in a wide variety of outer diameters described by gauge numbers. Smaller gauge numbers indicate larger outer diameters.[4] Inner diameter depends on both gauge and wall thickness. The following chart shows nominal inner diameter and wall thickness for regular-wall needles.[4][5] Thin-wall needles (not shown) have identical outer diameters but larger inner diameters for a given gauge.

https://en.wikipedia.org/wiki/Birmingham_gauge