I got it.Here is what POH says:
AUTOPILOT DISCONNECT (A/P DISC/TRIM INT) SWITCH When depressed, will disengage the autopilot and interrupt Manual Electric Trim (MET) power.
The button doesn’t work in the game in a correct way. A custom key can not be assigned to it. Will report a bug. I mean you can assign “Disengage autopilot” but it supposed to be a little more than that. The button should also stop ‘Manual Electric Trim’ (MET) from functioning. In case of malfunction the MET can be switched off and the analog trim wheel can be used instead.