They also use microscopic yellow dot patterns. Black and white only prints use a microscopic grey print pattern at the print boundary. The technique is a form of steganography. They aren’t tracking you btw. It gets used primarily to investigate fraud. Printer companies do it primarily because if they don’t, their brand will become associated with print related crimes. There are lists of printers that do not do steganographic serialization but those machines are almost entirely too poor quality to produce any convincing counterfeits anyways.