That's the global FX return. Normally don't change them because other songs & styles will be counting on the global returns to be at the default (50%) levels.
If your effects sends were all 0, then the amount of global returns wouldn't matter, but pretty much everything except bass requires reverb; conversely chorus is very easy to over-do, so if you set the global chorus return to 0, you probably wouldn't be missing much (generally lightly sprinkled on guitar, bass, electric pianos. Would sound absolutely awful on acoustic piano). For example, if you set both global FX returns to 0, nothing in the styles would receive any reverb or chorus; without reverb most instruments sound unnaturally dry.
But generally you don't have to worry about the global fx returns. Within the style, as you already noted, there are individual effects sends for each style/song channel because you don't send the same amount of reverb and chorus for everything.