+The O-RAN SC external call-out server allows behavioral extensions to be added to the A1 Simulator. It creates an external call-out server, which provides a RESTful API. A1 Policy operations, for some A1 Policy Types, can then be redirected to the external call-out server, supporting supplemental simulator behavior for those A1 Policy Types.
+
+**Note:** call-out server functionality is only available for *'STD_2.0.0'* version simulators.
+
+The external call-out server exposes a 'Callout server API' REST API. This internal API is invoked directly by the A1 Simulator, and is not intended to be used by any other client. The 'Callout Server API' is documented in `Callout Server API <./EXT_SRV_api.html>`_ and in OpenAPI YAML format: