Photoshop :: Replace Color Command Saturation...

For the Replace Color Command (not the Replace Color Tool), the meaning of the Saturation slider is clear only when the slider value is negative (reducing saturation). In that case the slider reading is the percent by which the (H-L) gap is to be reduced, where H and L are the highest and lowest RGB channel values. A reading of -100 means the gap is to be completely closed. Whatever the percentage closure, it is effected by decreasing H and increasing L in equal increments proportional to the slider setting.

When the saturation slider is set to a positive number (increasing saturation), however, the meaning is not easily unearthed. The incremental changes in H and L, although still equal and opposite in algebraic sign, are now a non-linear function of the slider and the initial H and L values, but I have not been able to nail it.

The middle RGB channel plays no role in either case. With H and L set, it is adjusted to keep hue constant.