From time to time Seiko launches Limited Edition watches. These are produced in very small quantities and each watch has a unique serial number engraved on the case back. E.g. if only 2000 of a particular limited edition model are produced each of these 2000 watches will have their unique number (from 1 to 2000) engraved on the back. These are usually in high demand because of their resale value. Normally you can sell a limited edition watch for much higher price even after many years since many of them are collector's items.