Final answer:
It is important to ensure the APU is AVAIL before shutting down an engine on taxi to maintain electrical power and system functionality.
Step-by-step explanation:
Before shutting down an engine on taxi, it is important to ensure that the APU (Auxiliary Power Unit) is AVAIL (Available) because the APU is responsible for providing electrical power to the aircraft when the engines are not running. When an engine is shut down, the electricity generated by that engine is lost, and if the APU is not available, the aircraft may lose all electrical power, including critical systems such as the navigation instruments, communication systems, and flight controls. Having the APU available ensures that the aircraft remains powered and the systems continue to function normally, reducing the risk of an emergency situation.