CALs are required by Microsoft licensing for all users or devices that access Windows Server Standard or Windows Server Datacenter. When a customer buys Windows Server Standard or Datacenter, they receive a server license that allows them to install the operating system on one computer. A server software license by itself doesn’t provide the license rights to allow anyone to connect to that computer, whether they work for the company or not. Instead, each user or device needs a CAL.
No CALs required for Windows Server 2012 or 2012 R2 Essentials or Foundation.
A small office with 15 users that access Windows Server 2012 R2 from multiple devices (desktops, laptops, tablets, phones) buys a server installed with Windows Server 2012 R2 Standard. They should also buy 15 Windows Server 2012 User CALs, one for each employee.
The server license gives you the right to install and use the server software. A CAL grants a user or device the right to access the server software. This structure offers license affordability to organizations of all sizes.