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The Devignetting slider removes the vignetting effect partially or completely by dragging it from 0 to 100%, where 0 means no vignetting elimination (the edges will be naturally darkened) and 100 mean full vignetting elimination (clean edges).
Devignetting 100% means full vignetting elimination (clean edges)
In addition to that, Devignetting can also take negative values (from 0 to -100) so you essentially can add darkening to the edges of the image, that can be interesting for stylistic purposes.
When Devignetting is disabled (left) the edges appears naturally darkened regarding the focal length of your camera.
A Devignetting of 100% (middle) means full vignetting elimination thus clean edges, while with a negative Devignetting (right) you can add darkening to the image edges