What does the Print check measure?

Every design is drawn in vector, then measured at print size: pure black lines (no grey), never lighter than KDP's 0.75 pt, nothing outside the safe area (the margins), and every white space a reader colors, its area in mm² and its width (the widest circle that fits), against the limits for the book's readers. The card says, for example, "40 of 40 designs measured at print size and passed: pure black lines of 3 pt (KDP needs 0.75 pt or more), spaces sized for bold and easy (adults) (25 mm² and 3.5 mm across), nothing in the margins."