Client Access Licenses (CALs) are needed when you would like your clients to access your server (s). You will see on many business websites a client log-in section which is a great tool for many types of businesses and are often used for customer service and even client communication. Of course, this incident has its own licensing requirements. Each device or user that plans to access the server requires its own license, whether it’s direct or indirect use of the server. (Note – a hardware partition or blade is considered a separate device to Microsoft so these must be counted as well).
There are two types of CALs – one for devices and one for users. Each device CAL permits one device, used by any user to access the server software. The user CAL permits one user, using any device – a combination of the two can be used, based on your needs.
Microsoft provides detailed information on which software applications require each CAL, which can be found here.
Tis a tricky game – this licensing!