All of our products have a design specification.
Print is usually grouped together and printed on large sheets with gaps in between. This is then guillotined down to the flat finished size of the product before folding. Crop marks are visible marks on the large sheets for where the guillotine operator should cut the sheets. When you submit artwork for printing with us you should omit crop marks, as they are added by our software and processes during production.
Why do i need to add bleed?
Printing machines and print finishing equipment, like other machines, have tolerances for the production of print products. For instance when a guillotine cuts paper to the finished size the paper may slightly move, and it may have already had variation in the exact location of the print on the sheets (from fractionally different paper sizes, moisture and many other variables).
So that is effectively invisible to the reader of the document we need to create a version of the print that allows for this tolerance. Without the tolerance the guillotine may cut just outside of the finished page and so leave a white line – that would really draw the eye – along one side of the document. This is avoided by extending the colour and photos by 3mm across the expected cut line. Then if the cut is, say, 1mm to the left the page still contains the colour or photo to the edge of the page.
The same reasons we require bleed to extend the page requires us to to the mirror of that on the inside of the edge of the page. The safe area is usually 5mm inside of the finished page (8mm from the edge of a PDF that includes 3mm for the bleed). That allows the tolerance for the printing machines, but more than that, if the page is offset to the left by 1mm then the reader will likely not notice as the page will still look balanced.