In fact there is 2 ways for using “mask”
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“traditionnal”, with “merge” (I call “blend”), image from mask, and original image. in this case you must use, all sliders - blend, radius, chroma mask, gamma mask, slope, and of course 1, 2 or 3 curves L(L), C(C) , LC(H)
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specific Rawtherapee, in this case, as says above by @sguyader , the mask serves to increase the selection capacity by artificially playing on the deltaE.
The image is not change at all, only “deltaE”.
In this case, you must use : 1 ,2 or 3 curves, radius…
If you use the others sliders, I recommand very very small values, because you change the image - of course , its perhaps the goal - but be carefull.
jacques