A Windows Server Client Access License (CAL) is a license that gives a user or device the right to access services, such as printing or access to a file share, from a server running the Windows Server operating system (OS). Microsoft offers device CALs and user CALs. A user CAL must be purchased for every user who accesses the server to use services. This is a more common licensing arrangement and grants each user the right to access server services from several different devices. A device CAL must be purchased for each device that accesses the server and is appropriate for machines that are used by several users, such as in a call center.