Scratch & dig specs are expressed as two numbers SSS/DD where SSS is the maximum scratch width, in decimal mm, and DD is the maximum depth also in decimal mm.
For example 120/20 means scratch widths not exceeding 0.120 mm and depth not exceeding 0.20 mm.
Items that fail the test may well finish up at SurplusShed or similar.
While a surface may look well polished to the naked eye, under a microscope it’s a whole different story.
Shining a laser through the glass will also highlight scratches.
One application where these numbers really matter is laser optics, where scratches will cause really bad scattering and with intense beams an imperfection will cause localise heating in the glass and quite likely damage the glass.
For telescopes it really only influences the scattered light ie extreme contrast could be compromised, if say you were hoping to spot Rigels companion star.
