Dust jacket.

Mixam describes a lightweight, removable paper cover with a front, back, spine and two folded flaps, and prints jackets for hardcovers only. PrintNinja says it is printed on one sheet, built as a single spread slightly larger than the book: 0.27 in added to the page height, 0.3 in to the page width, 0.04 in to the spine for the jacket paper, flaps of at least 2.83 in (71.9 mm) and 0.125 in of bleed. Mixam works in millimetres: 7 mm added to the front and back widths, 6 mm to the heights, flaps of 60 mm for books under 200 mm wide and 80 mm for wider ones, and 3 mm of bleed. The flap minimum is shorter at Mixam for a narrow book than at PrintNinja, so the template of the printer you use decides. Both are book printers, not print-on-demand marketplaces, and the Prep books guide found no jacket file specification on the platform pages it read. The jacket's spine comes from the spine width of the book plus a paper allowance, so a change in page count or paper changes the file.