Nope, and I don't believe there really is one, since it would depend on the process used (grit blast? lapping? buffing with a cotton buff?). Realistically, I think you could expect about an 8 u-in finish, plus or minus, using 220 mesh grit/polish. A bit higher if used in a tumbler or vibratory deburr process.
The customer may be refering to the grit size in the grinding wheel used for fabrication. I would expect a 220 grit (US mesh size) diamond wheel to produce a 32 microinch or better (probably no better than 20 microinch) finish on fused quartz.
Further to my post on a fixed abrasive wheel, I have attached a curve showing surface roughness vs. abrasive size on BK7 glass. This glass is a little softer than fused silica but results should be similar.
Source:
Zschlommer, W. Precision Optical Glassworking A Manual for Craftsmen and Designers. SPIE vol. 472. trans. Sachdev and Maxwell. 1984.
Why don't you simply ask the customer what it means. It sounds like they may have a standard setup for this finish.
At one time we were making a part with wrinkle paint surface. The customer actually supplied paint samples showing acceptable finish and unacceptable finish.