Channel Masks are the legacy method for creating Luminosity Masks in Photoshop, but can still be a good visual method for viewing some of the potential in Luminosity Masks.
When Luminosity Masks were first created in Photoshop, they were created in the Channels panel in Photoshop. Here we would create the Luminosity masks through a series of keyboard commands, restricting our selections until we achieved the desired result. With each keyboard command used to restrict the selection we would cut the tonal range selected in half or by 50%. If you have been using Luminosity Masks for some time you will have seen terms such as Lights 1, Lights 2, Lights 3 and so on, these were designations for a restriction in tonal range. The tools available now have superseded these tools, but they can still be a good visual method of seeing masks.
Pressing the Channel Masks button you will have the ability to create 6 masks in each of the three tonal ranges, Lights, Darks and Mid-Tones. These will be created in the Channels Panel, you can then go into your channels and view the masks. If you want to use one of the masks you Ctrl Click (PC) or CMD Click (Mac) to load up your selection, then move back to your layers to create and adjustment layer or any other method you’d like for using a mask.
When you are done using these masks in the channels, do not forget to go back into the Channel Masks button and delete them. If they are left in the channels the size of your Photoshop file will grow exponentially. With the use of the adjustment layers in mask creation today, we can create any of these masks and an infinite number of additional masks, so these are unlikely to be used often.
The video below will show how to use Channel Masks in more detail.