Final answer:
When an aircraft's APU green AVAIL light and EXT PWR blue ON light are illuminated, both power sources are available. However, external power typically takes precedence and is likely powering the aircraft's systems, conserving APU resources.
Step-by-step explanation:
When the APU green AVAIL light is ON and the EXT PWR blue ON light is illuminated on an aircraft, it indicates that both the Auxiliary Power Unit (APU) and the external power source are available to power the aircraft's systems. If both lights are on, it typically means that both systems are connected and ready to supply power, but the actual source that is currently powering the aircraft can depend on the specific aircraft's power management system and the procedures followed by the ground or flight crew. In many cases, priority may be given to external power when it is available to save on APU fuel and operating hours. Therefore, when both are available, external power is likely the one actively supplying power to the aircraft's systems, unless the crew has specifically selected APU power for operational reasons.