Work out artboard, bleed and safe area for a print job.
3 mm is standard in most of the world. US printers often ask for 3.175 mm, which is an eighth of an inch.
Set the artboard to 216 × 303 mm
Bleed exists because cutting is not perfect. A stack of paper shifts by a fraction of a millimetre under the guillotine, and without bleed that shift shows as a white sliver along one edge. Extending the background past the trim line by 3 mm gives the blade somewhere to land. The safe area is the same problem from the other direction: text sitting closer than 5 mm to the trim looks crowded on a good cut and gets clipped on a poor one. Export as a PDF with the bleed included and trim marks switched on rather than sending an image the size of the artboard, since a printer needs to know where the trim line actually is.
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