Use the Connect As dialog box to select the method that IIS uses to access content from the physical path that was specified for the site, application, or virtual directory. The physical path can be a path to a directory on the local computer or to a directory or share on a remote computer.
UI Element List
Select this option to provide credentials for a specific user account that has access to the physical path.
Opens the Set Credentials dialog box from which you can type the user name and password for the user account that has access to the physical path. This user will be used to access all of the content in the specified physical path. This button is available only when you select the Specific user option in the Connect As dialog box.
Application user (pass-through authentication)
Select this option if you want to use pass-through authentication. With this option, IIS uses the credentials of the requesting user to access the physical path.
For anonymous requests, IIS uses the identity configured for the anonymous authentication to access the physical path. By default, this identity is the built-in IUSR account.
For authenticated requests, IIS uses the authenticated set of credentials from the requesting user to access the physical path. Make sure that the application pool identity that serves this application has Read access to the physical path, so that the authenticated user can access the content in the physical path.