A clipping mask lets you use the content of a layer to mask the layers above it. The masking is determined by the content of the bottom or base layer. The non-transparent content of the base layer clips (reveals), the content of the layers above it in the clipping mask. All other content in the clipped layers is masked out.
You
can use multiple layers in a clipping mask, but they must be successive
layers. The name of the base layer in the mask is underlined, and
the thumbnails for the overlying layers are indented. The overlying
layers display a clipping mask icon .
The Blend Clipped Layers As Group option in the Layer Style dialog box determines whether the blending mode of the base affects the whole group or just the base. (See Group blend effects.)
Hold down Alt (Windows) or Option (Mac OS),
position the pointer over the line separating two grouped layers
in the Layers palette (the pointer changes to two overlapping circles ), and
click.
In the Layers palette, select a layer in the clipping mask, and choose Layer > Release Clipping Mask. This command removes the selected layer and any layers above it from the clipping mask.