question about PID multiwii

excuse me

I want to develop quadcopter with multiwii, but I have a constraint on PID. I have some questions: 1. how to translate the PID program on Multiwii? means the PID formula on multiwii was like what? 2. on the codding where I can find the settings for sensor calibration 3. there anything you can tell about the program flow or does anyone have a flow chart of the program multiwii?