What is a Fuel Over Pressure Protector?
It is possible to overfill a wing. You fuel the aircraft when its wing is cold. Later as the aircraft sits on the ramp the outside temp increases causing the full to expand in an already full wing. So they have the overflow system dumps the excess fuel on the ramp. Its usually not a lot of fuel just a few gallons.
  It also prevents the fueler from over fueling the wind. It can sometime dump 50 gallons or more if the fueler it not paying attention.