The tools in Capture One typically apply global adjustments to the whole image, however, there are times when you want to perform corrections to a specific part of the image. For example, you might want to simply sharpen the eyes in a portrait, fix some blemishes or apply a graduated filter effect.
There are several ways to create a mask: painting with a brush or by creating a Linear Gradient Mask / Radial Gradient Mask / Healing Mask / Cloning Mask that is non-destructive in nature and therefore can be readjusted easily at any given time. They can all be accessed from either the Layers tool, the Cursor toolbar, or by using their assigned keyboard shortcuts.
The most versatile is the brush tool called Draw Mask. This lets you apply adjustments to specific areas in an image by painting a mask over the desired areas. The Linear Gradient Mask and the Radial Gradient Mask, on the other hand, work by creating a gradient area that you can always readjust. The Healing Mask and the Cloning Mask are connected with the source point(s) which can be moved around freely. All of those masked areas are applied in a non-destructive fashion, meaning that you can always change the size, shape, rotation, feathering, and position of the mask after the fact.
Once a mask has been created, you apply the image adjustments in the same way as you would when making global adjustments. You can make any combination of image adjustments from different tools on the same Layer mask, so nothing prevents you from tweaking both Exposure, Clarity, and Color Balance to one specific layer mask.
If you need to apply a number of localized image adjustments to different parts of the image, just create a new layer with a separate mask.
NOTE: You can create up to 16 individual layers per image.