MS Title Group:
An Exchange Server Standard user client access license (CAL) authorizes one user to access Exchange Server Standard or Enterprise edition from any device. For example, an employee might access the server from a computer at work and another at home.
This license does not require media, license keys, or setup codes.
CALs, ECLs, and management licenses (MLs) offered through TechSoup are always for the currently offered version of the server software. However, these licenses can also be used with earlier versions of the server software.
Under the Software Assurance program, you have the right to install any new release of products covered in the agreement during the term of your coverage.
Product restrictions can be found under Microsoft's eligibility guidelines.