A Client Access License is Microsoft's name for various "keys" which must be purchased and enabled to allow various functionality on Windows NT based servers.

Terminal Services

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Terminal Services requires a license for the server itself in all cases, just as with any other Windows install. Windows XP has a built-in one-user terminal services server license, and 2000 Server and 2003 Server have "remote administration" modes which function similarly to XP in this regard, though 2003 server supports up to two connections in this mode.

Windows Server versions prior to 2003 do not necessarily require the use of specialized Terminal Services CALs; rather, clients which are of at least the same version of NT are allowed access automatically. For example, NT4 clients may connect to NT4 Terminal Server but not 2000 or later; Windows 2000 or XP clients may connect to NT4 or 2000 terminal servers.

Windows Server 2003 requires separate CALs for all clients in terminal server mode, regardless of the OS of the client.

Microsoft