From VFXPedia

Effect Masks

Fusion's Effect Masks are used to restrict the effect of a tool to a specified portion of an image. Masks can be polylines, basic primitive shapes, paint strokes or bitmaps from other tools.

To add an effect mask to the currently displayed tool, click on the button for the desired type of mask in the view toolbar or right-click in the display view and select Effect Mask from the context menu.

The toolbar buttons can only be used to add effect masks. Garbage and pre- masks are added via the view's context menu.

For more information, see the Effect Masks chapter.