Each machine with an installation of ArcGIS Server requires an authorization file. There are several ways to register ArcGIS Server. You can visit My Esri to obtain an authorization file (by email, fax, phone, or mail) before proceeding with the installation. You can also register ArcGIS Server using the Software Authorization Wizard that launches after the setup installation completes (this is the recommended method).
Registration requires registration numbers similar in format to ABC123456789. Visit My Esri for more information.