CAL
full spelling ： Client Access License
A Client Access License is cost-effective options for licensing the Windows Server family of products. One server license is required for each copy of the server software installed. In addition, a Windows Server Client Access License (CAL) is required for each user or device (or combination of both) that accesses the server software.
There are two types of Windows Client Access Licenses: device-based and user-based. They are also known as Windows Device CALs and Windows User CALs.