PDF page boxes.

Prepressure lists up to five. The MediaBox is the page size and is always present; for print work it is usually larger than the trimmed page, to leave room for bleed and marks. The TrimBox is the intended size of the finished page, and imposition software uses it to place pages on a press sheet. The BleedBox is the region to which the content is clipped in production, usually 3 to 5 mm larger than the TrimBox, though many prepress systems let the operator set bleed and ignore it. The CropBox is what a viewer displays and prints, and Prepressure says the GWG industry association recommends not using it. The ArtBox is the least used, sometimes marking a safe zone. For PDF/X the same page says PDF/X-1a and PDF/X-3 files need MediaBox, TrimBox and BleedBox, and PDF/X-4 needs the MediaBox plus either a TrimBox or an ArtBox, not both; its X-4 page adds that a file printed with bleed must define a BleedBox larger than the TrimBox and smaller than the MediaBox. Both pages are one reference site's summary; the ISO text was not read. A viewer shows the CropBox, so what appears on screen is not necessarily the TrimBox.