The User CAL allows you to purchase a CAL for each user who connects to the server to use services such as file storage or printing, regardless of the number of devices they use.
Purchasing a User CAL may make more sense if your company’s employees require roaming access to the corporate network from multiple devices, or from unknown devices, or if your organization simply has more devices than users.
An external user is someone who does not have employee-level access to your company’s or your affiliates’ networks and is not someone to whom you provide hosted services.
An EC license assigned to a server allows access by an unlimited number of external users, as long as the access is for the benefit of the licensee rather than the external user.
Do I need client access licenses?
If your organization’s workstations are networked, you most likely rely on network server software to perform certain functions, such as file and print sharing. A Client Access License (CAL) may be required to legally access this server software.
A CAL is not a software product; rather, it is a license that grants a user access to the server’s services. So yes you need a client access license.