Regions and masks
Ordinarily, a tool affects your entire photo. Using regions to create masks, you can restrict a tool’s effect to just specific areas of your photo. A mask is the set of regions for a particular tool.
Additionally, each region has an adjustable area that does feathering thus eliminating sharp edges between where a region is and it isn’t.
The control points of a region determine its shape and are used to adjust it. The points are marked by small, white circles. For example, the region shown on the right has three control points. (There is no significance to the order.) There is no limit to the number of control points a region may have.
To learn more about working with regions, click a link below.