http://technet2.microsoft.com/WindowsServer/en/Library/509ada1a-9fdc-45c1-8739-20085b20797b1033.mspx

Extending Your Active Directory Schema in Windows Server 2003 R2

Windows Server 2003 R2 was designed to be as compatible as possible with previous versions of the Windows Server operating system. This compatibility applies to the Active Directory schema that is used by Windows Server 2003 R2. Unless you are planning to install three particular new features or to use a computer running Windows Server 2003 R2 as a domain controller, it is not necessary to change your current Active Directory schema. However, if you plan to install any of the three new features or to use a computer running Windows Server 2003 R2 as a domain controller, you must first prepare Active Directory by extending the schema to accommodate the needs of these features. The following features require an extension of the Active Directory schema:

Distributed File System (DFS) Replication Service

Print Management Console

Identity Management for UNIX

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