Masks vs Photo Regions

A mask is used to protect part of a photo from a photo operation. For example when using the Content-aware Scale/Zoom tool to stretch or zoom a photo, you often want to 'protect' part of the photo to stop it being distorted by the operation. You use a mask to mark that area of protection.

Conversely the Photo Region tools are used to select areas of a photo upon which you wish to perform a photo operation. This might involve a Photo enhance operation to brighten or tweak the color of a particular area of a photo, or you might want to just perform a simple crop, or cut and paste part of a photo.

Regions and masks are actually interchangeable and the difference between them is only really one of presentation. A region identifies the part of a photo you want to operate on, while a mask works the other way around and identifies part of an image you don't want to operate on (the masked area is protected from the operation). And in fact if you have drawn a region using a region tool and you switch to the mask painter tool, the region becomes a mask. And similarly if you start with a mask and switch to a region tool, the mask becomes a region.

Masks have the advantage of being able to show levels of 'softness' whereas regions are indicated only by a hard outline. So if, for example, you use the Feather Tool on a region, you won't see any change to the region as you apply the feathering. But when you use that feathered region to enhance your photo, you'll see that the enhanced area is indeed feathered. If you use a mask instead, the pink fill of the mask shape will show the feathering.