How do external tanks differ from main tanks?

External tanks are specifically designed to be attached to the outside of the aircraft. This design allows for additional fuel capacity without compromising the space inside the fuselage, which is typically reserved for primary functions, including passenger or cargo space. By being external, these tanks can enhance the range and operational capability of the aircraft without altering the main tank's configuration or function.

In contrast, main tanks are internal components of the aircraft's structure that hold the primary fuel supply. They are built into the wings or the fuselage and are integral to the aircraft's balance and aerodynamics. External tanks are usually removable, enabling flexibility for different missions, whether short or long-range.

The other options do not accurately describe external tanks: they do not store more fuel than main tanks, they are specifically external rather than internal, and they indeed supply fuel as they are designed to feed into the aircraft's fuel system when in use.