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 Small Business Server 2011 Standard Add 5 User CALs
Processor: One 1.4 GHz (x64 processor) or one 1.3 GHz dual-core; or faster (Supports up to 4 Sockets)