Microsoft Exchange Server Standard User CAL (Includes Software Assurance)

This standard user client access licence (CAL) authorises a user to
access Exchange Server from any device (for example, when an employee
accesses the server from a computer at work and another at home). It is
valid for Exchange Server 2010 Enterprise and Standard editions, as well as previous versions of Exchange Server.

Enterprise Exchange Server CALs (user and device) are also available, providing access to additional features not covered by the standard CAL.

Device CALs
are also available. A device CAL authorizes one computer to access
Exchange Server, regardless of the number of users (for example, on a
shared workstation).