BVM Go Fly Gold F-18C Assembly And Operating Manual page 31

 Install the fittings to your overflow/taxi tank. Use an overflow tank while fueling to prevent
spillage and to ensure fuel tanks are full before flight.
Use BVM Overflow tank Part #BVM6047.
Below, the overflow/taxi tank is connected. The vent plug is installed.
BVM ®2021 30 10/2021