You can apply a layer mask to permanently delete the hidden portions of a layer. Layer masks are stored as alpha channels, so applying and deleting layer masks can help reduce file size. You can also delete a layer mask without applying the changes.

1. Click the layer mask thumbnail in the Layers palette.

2. To remove the layer mask after applying it permanently to the layer, click the Delete icon  at the bottom of the Layers palette, and then click Apply.

3. To remove the layer mask without applying it to the layer, click the Delete icon at the bottom of the Layers palette, and then click Delete.

   You can also apply or delete layer masks using the Layer menu.

   Note: You cannot apply a layer mask permanently to a Smart Object layer when deleting the layer mask.