In addition to per user and per device licenses with Windows 2003 Server, there is a third option called External Connector Licenses. Because per user and per device client access licenses (CALs) have to be assigned to a specific user account or device, this sometimes makes it difficult when companies would like to grant access to their servers to partners and/or clients. Whenever systems are extended to unknown users, an External Connector License can be purchased.
These license options are available for all Microsoft products, but one important thing to keep in mind – this isn’t a cost effective way to grant your employees access without accounting for them. Microsoft has very strict guidelines for ECLs and if they are used by employees, you will no longer be in compliance.