Reference axis for Euler angles of Simulink's FlightGear hookup?

I am trying to hook up my UAV Simulink simulation to Flight Gear. To do this, I am using Aersospace Blockset's FlightGear Preconfigured 6DoF Animation block. The documentation on it does not say what the reference axis is for the roll, pitch, and yaw angles. It only specifies radians. Is the reference frame due North counter clockwise? Or something else?

Source: https://www.mathworks.com/help/aeroblks/flightgearpreconfigured6dofanimation.html