Subtraction Masks are a method built into the panel that allow you to create your own mask when you cannot find or create a mask using traditional Luminosity Mask selections
Subtraction Masks use Luminosity Masks in their creation. When creating a subtraction masks you need to make a couple of choices. Firstly you choose the tonal range that you want to subtract from, so this is your main selection covering the tonal area that you want to work in. These tonal ranges include, all tones, lights, darks and all of the mid-tones. You then select the tonal area that you want to subtract and these are either lights or darks.
Once you make these two selections you that press execute in the panel. Two masks will appear in succession allowing you to make adjustments to the tonal ranges. The first mask that appears is the Subtract from mask, you simply modify it affect the tonal range you want to work in. The second mask that will appear is the area that you don’t want effected in the tonal range you selected. Once you press OK on the second mask the panel will complete the subtraction and leave you with a loaded selection that you can use with other tools in the panel.
If you choose to subtract from “All Tones” this will not appear as a mask that you can adjust, because you are simply subtracting from all tones in the image.
The video below will show how to create subtraction masks in more detail.