The Used By tab lists the classes that the security policy is assigned to. The default security policy is assigned to instances based on the class.
This read-only list includes document, folder, and custom object classes.
NOTE To make changes to this list, see Assign a security policy or change the current assignment. The Used By property page does not list the documents, folders, or custom objects to which the current security policy was assigned directly, as opposed to being the default for the object's class.