Setting a print back from the glass protects the art without the need for a mat. Generally, no mat is shown, the print being flush with the edge of the frame. Box frames are mainly used to set a print back as they allow for space between the backing board and glass.
While the print is usually set back the whole depth of the frame, it is possible to set back a portion of this. Photographic and strong graphic works suit a set back frame particularly well.