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Bleed is like an "extra bit" of your background picture or color that goes beyond the edge of the page. We need this if you want your background to print all the way to the edge without any white edges showing. When your item is cut to its final size, this extra bit gets trimmed off.
We ask for a 3mm bleed on all documents, except for cover files for Case Bound Books. For those, we need a 20mm bleed to wrap around the case. Our templates show where all of this goes.
It's easiest if you add bleed before you upload your print-ready PDF file. Some programs, like InDesign, Illustrator, and Photoshop, let you do this.