Pos PI controller
It is essential that your heading works perfectly for GPS position hold or navigation to work. Ensure that the magnetometer calibrated correctly and not influenced by magnetic fields from power wiring.
Tuning the Pos PI controller
The Pos PI controller is used to determine how fast the multirotor must go to reach it's position hold location. The error between the current location and the hold location together with the P and I settings of the Pos controller determine how fast the multirotor will try to fly to reach it's position hold location.
It is suggested that you start with the default PI parameters and set up Pos Rate first in order to achieve smooth attitude control based on the Pos PI controller commanded speed.
With larger P values, the flight controller will command a higher speed towards the desired location.
Pos I is used to overcome any wind pushing the multirotor away from the target hold location. The longer the error persists, the more the I term will 'wind up' the output to compensate for the prolonged error. Setting I too high will cause oscillations or overshoot possibly leading to loss of control.