About Client Access Licenses (CALs): In Per User or Per Device mode a separate CAL is required for each user or device that accesses or uses the Microsoft Small Business Server 2003 software on any server. You may reassign a Windows CAL from one device to another device or from one user to another user provided the reassignment is made either (a) permanently away from the one device or user or (b) temporarily to accommodate the use of the Windows CAL either by a loaner device while a permanent device is out of service or by a temporary worker while a regular employee is absent.
By having two types of Windows CALs you are able to use the model that makes sense for your organization. For example purchasing a Windows User CAL might make more sense if your company has a need for employees to have roaming access using multiple devices. Windows Device CALs may make more sense if your company has multiple-shift workers who share devices.