RDS CAL is a license that allow the remote access to a server. CAL refers to "Client Access License". In the 2016 edition there are two CAL versions - User and Device CALs. The type you need depends on whether you want to provide a user or a device with access to the server.
Remote Desktop Services 2016 (RDS 2016) is an important part of Microsoft's licensing. Users and devices can connect remotely to the server with the help of RDS. You require RDS CALs to do this. CAL stands for client access license and it facilitates the connection between the user/device and the remote server. Remote Desktop Services, also known by its previous name - Terminal Services, provides a layer of flexibility which is an advantage for many companies. The primary factor driving businesses to integrate Remote Desktop Services 2016 into their standard operating procedures is the flexibility of this Microsoft license. RDS CALs can provide users and devices with permission to visit the server. See below for which Microsoft servers you can use RDS 2016.
Remote Desktop Services 2016 with Server
Microsoft SQL Server
Microsoft Sharepoint Server
Microsoft Exchange Server
Microsoft Windows Server
Note that without standard server CALs, you cannot use Remote Desktop Services with a server!
RDS 2016 User CALs vs. RDS 2016 Device CALs
As with standard server CALs, you can choose between RDS 2016 User and RDS 2016 Device CALs. As the names suggest, the User CALs provide remote access for users to the server and Device CALs facilitate the remote connection between a device and the server. If you are not sure which RDS 2016 CALs to choose, you can take the number of employees and devices that would need remote access to the server into account. A general rule of thumb is to license the ones that are fewer. For example, if 7 employees would remotely connect to the server with the use of 19 devices in total, you can purchase 7 RDS 2016 User CALs instead of 19 RDS 2016 Device CALs. The only important thing is to remember that RDS CALs do not substitute standard server CALs.