A clipping mask is an object whose shape masks other artwork so that only areas that lie within the shape are visible—in effect, clipping the artwork to the shape of the mask. The clipping mask and the objects that are masked are called a clipping set. You can make a clipping set from a selection of two or more objects or from all objects in a group or layer.
Object-level clipping sets are combined as a group in the Layers panel. If you create layer-level clipping sets, the object on top of the layer clips all of the objects below it. All operations that you perform on an object-level clipping set, such as transformations and alignment, are based on the clipping mask’s boundary, not the unmasked boundary. Once you have created an object-level clipping mask, you can only select the clipped content by using the Layers panel, the Direct Selection tool, or by isolating the clipping set.