Pressing the MACH/IAS momentary action pushbutton toggles the selected value shown in the speed window between THR and PCH.

The correct interpretation of the function of the MACH/IAS momentary action pushbutton is that it does not toggle the selected value displayed in the speed window between THR (Throttle) and PCH (Pitch Hold). The pushbutton's role is to switch the indicated airspeed (IAS) and Mach number as displayed in the speed window, not to alternate between two distinct modes or values.

Understanding how this toggle works is crucial for maintaining proper flight mode and ensuring accurate speed management. The speed window is intended to reflect either IAS or Mach depending on the current operational needs, but the specifics of the functionality mean it does not directly switch between THR and PCH values. This distinction is important for correctly managing and interpreting the aircraft's speed control systems.