Range mask
A range mask refines an existing mask in Lightroom based on colour, luminance or depth. So you limit a gradient to just the blue sky, for example, or a brush to just the dark parts. It makes sure your adjustment lands exactly where you want it.
You add a range mask within another mask, like an extra filter on top. With the colour range you click a colour in the photo and set how widely the selection extends. With luminance range you choose by brightness, ideal for hitting only the highlights or shadows. Tip: draw a rough gradient or brush first and squeeze it down with a range mask afterwards, that's faster and more accurate than brushing precisely inside the lines.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.