A license server must be activated to certify the license server and allow the license server to issue Remote Desktop Services client access licenses (RDS CALs). You can activate a license server by using the Activate Server Wizard in the Remote Desktop Licensing Manager tool.
There are three methods by which you can activate your license server:
Activate a Remote Desktop License Server Automatically
Activate a Remote Desktop License Server by Using a Web Browser
Activate a Remote Desktop License Server by Using the Telephone
When you activate the license server, Microsoft provides the server with a limited-use digital certificate that validates server ownership and identity. Microsoft uses an X.509 industry standard certificate for this purpose. By using this certificate, a license server can make subsequent transactions with Microsoft.
If a license server is not activated, the license server can only issue temporary RDS Per Device CALs, which are valid for 90 days, or RDS Per User CALs.