* Select a specific tank: This might be necessary to balance fuel load, to use fuel from a particular tank first (e.g., if one tank is contaminated), or to avoid running a tank dry.
* Select multiple tanks: This allows the aircraft to draw fuel from both tanks simultaneously.
* Select "OFF" or a "BOTH" position: The "OFF" position shuts off fuel flow completely. "BOTH" (or a similar designation) draws fuel from all available tanks.
The exact positions and labels may vary between aircraft types, but the underlying function remains the same: to control the fuel source for the engine(s).
