Use the new Mask Reference option to restrict the area of effects that is applied to a layer. The Mask Reference property is found under the Compositing Options under each effect in the Timeline panel.
Click the '+' sign under the Compositing Options in the Timeline panel and choose a mask from the Mask Reference menu.
While creating a mask reference, keep in mind the following considerations :
When a mask is applied to a layer, a dynamic stream is created under the Masks options which shows the list of masks applied to a layer. When a new mask is created, this list is automatically updated.
Changes to a mask that is referenced by an effect will change where the effect is composited. For example, you can feather the mask or use per-vertex feathering to change the edge of the effect composite. Changing the opacity changes the overall intensity of the effect per for every mask.
You can also use mask modes and combine multiple masks to apply an effect only to the area of intersection of masks.
For more information, see the Mask modes section.
An effect mask cannot be used to modify the alpha channel of a layer. Hence an effect mask cannot be used as a layer mask.
For more information about layer masks, see the Masks and alpha channels section.