It is almost similar to the Curves that we see in the Image>Adjustment menu.
If you see the graph, the y-axis is divided into 4 parts. Each part represents a region. Upper parts demostrate the light and lower parts demostrate the shadow.
Highlight: It affects only the upper regions
Light: It mainly affects the upper two regions but slightly affects lower two regions as well
Darks: It mainly affects lower two regions but slightly affects upper two regions as well
Shadows: It only affects lower two regions
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