Gauge
One hundredth of a thousandth of an inch — exactly 0.254 microns. The sack trade rounds it to four gauge per micron, which runs 1.6% light.
Also called: gg
The rounding is where the tidy numbers come from: 120 gauge for 30 µm, 200 for 50, 500 for 125. Strictly those are 30.5, 50.8 and 127.
It is a rounding of one scale, not a second scale. Stretch film uses the same conversion. See the micron and gauge converter.