A clipping group is a group of layers to which a mask is applied. The bottommost layer, or base layer, defines the visible boundaries of the entire group. For example, you might have a shape in the base layer, a photograph in the layer above it, and text in the topmost layer. If you group them, the photograph and text appear only through the shape outline in the base layer; they also take on the opacity of the base layer.
You can group only successive layers. The name of the base layer
in the group is underlined, and the thumbnails for the overlying
layers are indented. Additionally, the overlying layers display
the clipping group icon .