To use this endpoint, you must have read access to the given namespace. If no namespace is specified, or if the specified namespace is %SYS, you must have read access to the default namespace (USER). Note that you can set the default namespace to a different namespace; to do so, set the global node ^%SYS("REST","UserNamespace") equal to the desired namespace.
You must also have read access to the database that contains the dispatch class. The dispatch class namespace and the endpoint namespace need to be the same (except in the case of a dispatch class in a package starting with %, which is available to all namespaces).