[GPSNavX]

In the configuration you provide, only the NMEA data that is received on your Keyspan (1) serial port would be sent to a repeat serial port (if one is chosen). The reason MacENC does not filter out any NMEA data in the repeat NMEA data stream is MacENC has no idea what it is sending the NMEA data to. It could be an autopilot. It could be a RADAR. It could even be another Mac. Similar to how your AIS receiver passes through the GPS NMEA data. It would be possible to limit the repeat port data to only the GPS NMEA data MacENC processes, but I am not seeing the reason for doing so. Any instrument (autopilot, RADAR, etc.) that receives NMEA data only looks at the NMEA data it needs and ignores the rest. So the filtering you ask for occurs by default. "Autopilot Only" is provided as means to avoid a feedback loop. That is instead of repeating the NMEA data only the manufactured NMEA data is sent. This is because sometimes the repeat port is the same as the primary GPS port.