It assigns a serial number/key  or  a registration number/key 

                 to access the file, document,...    forever.

Best against crackers, hackers ...

                  When a user try a certain file, document,     **       

                  on a Trial-Shareware basis, then comes a time to own /keep it.

                  To reserve the right for users in buying such a property, a certain

                  key is assigned. Each user has a different key of which no user product

                  is capable of being an owner on himself. By this one guarantees business,

                  property claim, copyright, ...