RC_OPTIONS, 288 is used for telemetry passthrough over CRSF for the Yaapu telemetry script on the Radiomaster TX16S (no connection to Mission Planner at this point).
SERIAL2 was connected to the Crossfire Nano RX pins 3 & 4 for MAVLink over CRSF, where the PC with Mission Planner is connected via wifi to the Crossfire Micro TX. However that didn’t work for us in the end because of strict firewall settings (company laptop).
SERIAL1 is connected to a self-designed ESP32 telemetry module running dronebridge for the connection to Mission Planner, the comment is outdated.