This refers to various products available from professional locksmiths. If you buy such locks, you can be sure that no unauthorized duplicate keys will be made. Many businesses and institutions utilize such systems. If an employee’s key from a restricted set is returned upon their resignation, no rekeying is necessary (this can save thousands of dollars!). Other popular applications are neighborhood swimming pools, tennis courts, clubhouses and fitness centers. This allows the management to issue keys to those who pay their dues, certain that members cannot in turn make keys to share with other, non-paying acquaintances! NOTE: Stamping “do not duplicate” on an ordinary key is not very effective! Some locksmiths will require ID or a note on letterhead to duplicate such a key. But anyone can produce letterhead, and there is no way to verify authorization to duplicate. There is simply no way to control duplication of a key that is available at every locksmith shop, home improvement store, and shoe repair shop in the world. If you need to put an end to unauthorized duplication of keys at your home or workplace, you need to contact a professional locksmith for restricted key products!