Impersonation is independent of the authentication mode configured using the authentication configuration element. The authentication element is used to determine the User property of the current HttpContext. Impersonation is used to determine the WindowsIdentity of the ASP.NET application.
By default, ASP.NET is configured to use Windows authentication mode, which applies the Windows identity supplied by IIS to the User property of the current HttpContext object. This enables you to determine the identity supplied by IIS through the User property (the user Name is blank when anonymous identification is used), but does not use the supplied identity as the WindowsIdentity for the current page. The WindowsIdentity for an application is used when determining if the application has access to a particular file or network resource.
https://msdn.microsoft.com/en-us/library/134ec8tc.aspx
https://msdn.microsoft.com/en-us/library/ff650037.aspx