Question about px4 ekf2 setting

[dssdyx]

Hi! I have successed to let the nxtpx4 board work. But there is a little confusion about ekf2 estimator. In px4 user guider, EKF2 seems to require minimal sensors : a imu, a yaw estimator source(mag / vision),a height resource(baro). In nxtpx4, it only has imu and baro, no yaw source. But ekf2 can run well in this situation. It makes me really confusion. Would you like to tell me why it's ok? Thank you for your such brilian project!