The Windows Server 2008 External Connector (EC) license is an optional licensing alternative to address a specific customer scenario. If you would like to allow your business partners or customers to access your network, you have two licensing options:
Acquire Windows CALs for each of your external users.
Acquire External Connector (EC) licenses for each server licensed to run one or more instances of the Windows Server 2008 software that will be accessed by your external users.
An external user is a person who is not an employee, or similar personnel of the company or its affiliates, and is not someone to whom you provide hosted services using the server software. An EC license can be acquired for servers running Windows Server 2008 Standard, Enterprise, Datacenter, and Windows Server 2008 for Itanium-Based Systems editions. It is not applicable to Windows Server 2008 Web Server or Windows Small Business Server.
In addition to using the EC license with Windows Server 2008, you may opt to use it with an earlier version of the server software, provided the version is no earlier than Windows 2000. Due to technical incompatibility, the Terminal Server External Connector (TSEC) license cannot be used with earlier versions of the server software.