Page bleed area
A graphic, or similar, may bleed off (or be bled-out from) the cut edge of the page. Extending an image to the edge of the page is often a useful effect. If the image extends just to the edge of the trimmed page, inaccurate trimming could leave a white area along the outer edge of the image. Extending the image past the edge of the page then trimming to size avoids problems from inaccuracy when trimming.
Figure 7. Page bleed
Bleed region width : bleed property
Specifies the width of the bleed region for trimming. The bleed region extends outwards from the page box. By specifying a negative value to the margin box margin, the range of the block can be extended to the bleed region.