Manual says it has no influence on distorted profiles. The forums says it only effects clean to mildly distorted profiles (but just the output level), but you can clearly hear it’s changing a distorted profile that has a gain setting of 8. It’s somehow interactive with dist sense and you can alter where the focus is not unlike a mid band eq before the amp. For instance a lower clean sense below 0 with a dist sense above 0 almost scoops the sound a bit or thins it out. It’s not changing the out put volume. It’s changing the focus of the way the guitar hits the amp. You can even have dist sense on 0 and sweep around clean sense and you can clearly hear it affecting the tone, it’s more prominent on some profiles than others but try it on a Sinmix profile or a Big Hairy Profile and you’ll see what I mean.
Why doesn’t the manual address this?, it’s been 10 plus years. I can provide and EQ match screen shot if necessary.