Synchronization marks are black-colored rectangles located in the corners of the document in a distance from the corner of less than 15% of the page length (E.g. for A4 format, 1 cm from each side is an appropriate placement). All four synchronization marks must have the same size.
Special emphasis should be placed on printing quality of the synchronization marks because their possible damage (e.g. the whole square is not black or is missing) slows down the entire processing.