Windows CAL's

Microsoft currently offers either a device-based CAL (Device CAL) or a user-based CAL (User CAL). Server access CAL's are essential documents to comply with Microsoft licensing laws. They do not require activation but must be purchased according to how many users or machines will have access to the server in the company.

When using the server with remote desktop you need RDS CAL’s:

RDS CAL’s or Remote Desktop Services is a license you need to access a remote desktop on Windows Server. So, this license is only required if you intend to access the server remotely. To be compliant, you must license each user or computer that has remote access to the Server.

The RDS CALs, just as the normal CALs are divided into RDS Device CAL and RDS User CAL. Unlike the server access license, the RDS CAL has an activation key, and in volume licensing, one key can activate as many as the number of activations you purchase, making installation easier. Another very important point about RDS CAL's is that when licensing the Server, they do not replace the access CAL's, so if you are licensing your Server and need to work remotely to be in compliance you must buy the Server + CAL's + RDS CAL's.