A User CAL is designed to be purchased for every named user accessing your servers. For example, purchasing a User CAL might make more sense if your company has a need for employees to have roaming access using multiple devices.
System Requirements for Microsoft Windows Small Business Server 2011 Premium Add 20 User CALs
Processor: Minimum: Single processor with 1.4 GHz (x64 processor) or 1.3GHz (Dual Core) Recommended: 2.0 GHz or faster
Memory: Minimum:1 GB Recommended: 4 GB or more (refer to Windows Server and SQL Server Recommendations) Maximum: 32 GB
Disk Space Requirements: Minimum: 32 GB or greater Note: Computers with more than 16 GB of RAM will require more disk space for paging, hibernation, and dump files