Access-based Enumeration

Access-based enumeration displays only the files and folders that a user has permissions to access. It is a feature that was previously available as a downloadable package for the Windows Server® 2003 operating system (it was also included in Windows Server 2003 Service Pack 1). Access-based enumeration is now included in the Windows Server 2008 operating system, and you can enable it by using Share and Storage Management.

http://technet.microsoft.com/en-us/library/dd772681%28WS.10%29.aspx