Objects and pictures on your document pages can overlap each other. Normally you'll want them to print exactly as they appear on screen. To do this it's usual for the objects "underneath" to have areas corresponding to the overlying object cut out- or "knocked out"- of their separations, so that the overlying object can print clearly.

Sometimes, however, you may want to overprint rather than knock out. For example, you might want spot-color type to overprint an image to avoid registration problems on the printing press. When one object overprints another, no knockouts are made in the underlying objects.
RSG lets you specify knockouts or overprinting for objects throughout the document. You can also make individual knockout or overprint arrangements for particular objects, to give exactly the effect you need in print.