What exactly are Windows Server Remote Desktop Services CALs and what are they required for?
A Windows Remote Desktop Services CAL is a Client Access Licence, i.e. an access licence. It is required to licence the access and use of Remote Desktop Services by a human user or via a specific device. The Remote Desktop Services Client Access Licences (RDS CALs) are available in two variants. A Remote Desktop Services User CAL authorises a user to use RDS on any number of different devices. This form of licensing is useful if you only have a few RDS users in the company and they access your Windows servers remotely from different platforms - e.g. also from the home office.
A Device CAL authorises a specific device to use RDS and is particularly recommended if you have many different users who require remote access via a defined device. Microsoft's licence policy also allows the mixed use of user and device CALs, but this increases the administrative effort enormously. It therefore generally makes more sense to choose one of the two possible licensing options in advance, such as a Windows Remote Desktop Services User CAL.