Select the Add Unauthenticated Mask ... option to display the Add Unauthenticated Mask dialog box, which allows you to specify the permissions you are associating with the unauthenticated mask. If the menu item is dimmed, it means you already have this entry on your ACL. You can have only one such entry on an ACL.
The unauthenticated mask of an ACL specifies the maximum permission set that an unauthenticated request can receive. It is applied over the permissions granted to the requester by the ACL entries, and only permissions in both are actually granted.
Select the permissions you want by clicking on the desired permissions displayed in the Permissions box. Note that if you do not select any permissions, no permissions are granted and no access is allowed.