Masking is a non-destructive way to hide certain areas of an image. You can add multiple masks via layer groups which help you with in-depth training on Photography. You can also apply multiple masks to the same layer in order, adjust the transparency and so forth.
Learn more about Multiple Masks:
Working with multiple masks by Lynda
Apply Multiple Layer Masks to Single Layer in Photoshop CC 2015 by Design Easy
See also:
Free Online Multiple Masks Tools: