Windows Server Client Access Licenses (CALS) are used to authenticate users or devices to the Microsoft Windows Servers. Once CAL is required for each device or user (must select one) that accesses the server technology. These licenses can be used to authenticate clients to Windows Server Standard Ed., Web Ed., or Enterprise Ed.
TechShop FAQ
System Requirements
Windows Server 2008 Client Licenses are also backward-compatible with Windows 2003 Server installations. If you are purchasing installation media for Windows Server 2003 Standard, Enterprise, or Web editions, you will need to purchase 2008 client licenses.