Devignetting
Vignetting is an artifact that darkens an image around the edges due to camera optics. The Devignetting slider removes the vignetting effect partially or completely by dragging it from 0 to 100%, where 0 means no vignetting elimination (dark edges) and 100 mean full vignetting elimination (clean edges).
In the real world, the more wide-angle the lens is (for example, a focal length of 24mm), the more pronounced the vignetting will be. As you raise the focal length of the camera, the vignetting effect will be less pronounced. A focal length of around 80mm or higher will have almost no vignetting, no matter the Devignetting settings used.