Transparency masks let you keep a whole paint layer in Krita, but only have a selected area show.
How to add a transparency mask:
2. Click on the green plus sign (+) dropdown in the bottom left corner of the layers docker and choose "Transparency Mask".
Major advantages of using a transparency mask, is that it is a non destructive process. This means that you can always fine tune and edit what is visible and what is invisible. If you discover you've hidden part of your paint layer accidentally, you can always show it again just by painting white on your transparency mask. This makes for a workflow that is very tolerant to mistakes and using rough drafts that can be improved later.