Final answer:
The APU is typically started during the parking phase to provide power and air conditioning when the main engines are not running.
Step-by-step explanation:
The Auxiliary Power Unit (APU) is normally started on the ground during the parking phase. The APU's primary function is to provide power to the aircraft when the main engines are not running, which is particularly useful during parking, as it can supply electricity for onboard systems, air conditioning, and can serve as a power source to start the main engines. The APU is not typically used during takeoff, landing, or taxiing unless required for a particular operational reason.