Final answer:
The XFER-FILL switch, when moved to XFER, generally transfers fuel from the auxiliary tank to the main tank to manage the vehicle's or aircraft's fuel distribution for balance and efficiency.
Step-by-step explanation:
When the XFER-FILL switch is moved to XFER, it typically means that fuel is being transferred from the auxiliary tank to the main tank in an aircraft or vehicle. This is essential to manage the balance and distribution of fuel during different phases of operation. For example, in aircraft, it is important to maintain the center of gravity within specific limits for safety and efficiency, and transferring fuel between tanks is one way to achieve this. The switch does not fill both tanks or drain them; it moves fuel from one specific tank to another to ensure proper fuel management as needed during an operation