Each user or device accessing a licensed Windows Server requires a Windows Server CAL or a Windows Server and a Remote Desktop Services (RDS) CAL.

With the User CAL, you purchase a CAL for every user who accesses the server to use services such as file storage or printing, regardless of the number of devices they use for that access.

With a Device CAL, you purchase a CAL for every device that accesses your server, regardless of the number of users who use that device to access the server.