What is the correct `MAV_TYPE` for a onboard Visual Positioning System?

I’m building a visual positioning service that runs on onboard compute and sends VISION_POSITION_ESTIMATE etc messages to the flight controller. For the heartbeat emitted by this system, what is the correct value for the mav_type field? I can’t find detailed descriptions of the values anywhere. Would this constitude a “Companion Controller”? "Something else?