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Check page 5 on the autopilot document, inside c-172sp folder, S-TEC Autopilot Manual.pdf. There's a picture of what options to choose to assign a button or a key. I can't paste the image here but is something related to servos.
The text is as follows:
Autopilot Disconnect This button provides the pilot with a rapid-disconnect capability, used in situations where full control is being returned to the pilot permanently. Map your chosen button to the ‘Disco servos, whichever side is active’ function. Control Wheel Steering This button enables the pilot to temporarily override the autopilot servos and take control of pitch and roll (via the yoke or control stick). Releasing the button will return control authority to the autopilot. Map your chosen button to the ‘Control wheel steering mode’ function.
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