Bleed is simply where you size your artwork 3mm bigger than the required finished dimensions
The reason for this is once your artwork has been printed, it will need trimming to the finished dimensions - If you have a picture that goes to the edge of the finished page, it will need to be 3mm larger on all sides so that when the page is trimmed to size there is no white space at the edge of the page
In order for us to process your artwork as efficiently and professionally as we can, please supply the artwork for your paper parts with trim marks and bleed
Without them, processing your artwork may take longer, and ultimately the delivery of your finished discs may be delayed