This feature is available on webs hosted by web servers running SharePoint Team Services from Microsoft and FrontPage Server Extensions 2002 only.
Note When you enable unique permissions, a copy of the parent web's user accounts and roles remain with the subweb. You can then delete any accounts and roles that you don't want and add new ones as needed. The subweb will also retain the same user role setting for anonymous (guest) users as the parent web unless you specify a different role.