If the workstations in your organization are networked, you likely depend on network server software to perform certain functions, such as file and print sharing. To access this server software legally, a Client Access License (CAL) may be required. A CAL is not a software product; rather, it is a license that gives a user the right to access the services of the server.
With the User CAL, you purchase a CAL for every user who accesses the server to use services such as file storage or printing, regardless of the number of devices they use for that access. Purchasing a User CAL might make more sense if your company's employees need to have roaming access to the corporate network by using multiple devices, or from unknown devices, or if you simply have more devices than users in your organization