An @page rule defines a page box, but the page box may be printed on a page sheet that is larger than the page box. A common reason for this is so images and other content can extend up to the edge of the page box. A physical device such as a printer typically has a non-printable area around the edge of the page sheet where it is not capable of printing reliably, if at all. Printing the page box on a larger page sheet then trimming the page sheet to the size of the page box avoids problems with the non-printable area. Extending images, etc., into the bleed area outside the page box avoids problems if the trimming to size is inaccurate.

Crop and registration marks are printed outside the page box and are used as guides when trimming the page sheet to size and for checking that content printed on both sides of a duplex sheet is aligned correctly. Other information that may be printed outside the page box includes colour bars for checking colour fidelity as well as information identifying the page, its containing document, its version number, etc.

Figure 6. Crop mark terms