Any object can be used to mask another object. The opacity and lightness of the mask determines the opacity of the masked objects. A masked object will be fully opaque only at places that are: inside the mask path, where the mask has maximum lightness (i.e., white), and the mask has maximum Alpha. So to summarize:

To mask an object (or Group), select both the object to be masked and the object to be used as a mask. The mask must be above the object to be masked in z-order. Then use the Object Mask Set command. To unmask a masked object, select the object and use the Object Mask Release command. The mask is then restored as a regular object and is placed just above the formerly masked object in z-order.

More than one object (or Group) can be masked at the same time. Just follow the above instructions but include all the objects to be masked in the selection (with the mask on top). Inkscape will store one copy of the mask in the <defs> section of the SVG file for each masked object; thus, the masked objects can be edited separately.

When a masked object is selected, the Status Bar will display the type of object clipped along with the word masked. The bounding box of the masked object is the same as that of the unmasked object.