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.
Licensing software with CALs and MLs can be complicated due to the technical nature of server products and networks.
Microsoft offers a device-based CAL (Device CAL) or a user-based CAL (User CAL) for purchase.
In addition, an External Connector (EC) license is offered for some products as an optional alternative to address specific customer scenarios.
For managing devices, Microsoft offers Server MLs for managing server Operating System Environments (OSEs).
For managing non-server devices, Microsoft offers OSE Client MLs and User Client MLs. Before purchasing, you should visit the "How to Buy" section for each product and consult your Microsoft representative or local reseller regarding your specific licensing needs.
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.
Client Access License based on user With a Device CAL, you purchase a CAL for every device that accesses your server, regardless of the number of users who use that device to access the server.
Device CALs may make more economic and administrative sense if your company has workers who share devices, for example, on different work shifts.