What distinguishes auxiliary tanks from extended range tanks?

Auxiliary tanks are typically distinguished from extended range tanks primarily by their location within the aircraft. Auxiliary tanks are usually situated in less conventional areas, such as the wings or fuselage, and are designed to feed fuel directly into the main fuel system to be used by the engines. Their specific placement often serves to help manage the aircraft's center of gravity and overall aerodynamics during flight.

The other options suggest characteristics that do not universally define auxiliary tanks as compared to extended range tanks. While it's true that auxiliary tanks can supply fuel to the main tank, this feature is not unique to them and does not differentiate them effectively from extended range tanks. Similarly, while auxiliary tanks might have varying capacities, the defining characteristic is not strictly size, as both tank types can come in various sizes to meet specific mission requirements. Furthermore, it is possible for some auxiliary tanks to be filled during flight, but this capability does not apply to all tank designs, nor does it affect their classification. Thus, the defining aspect of auxiliary tanks is their location within the aircraft.