Final answer:
APU bleed air is used to feed the environmental control system, help in engine starting, anti-ice systems, and pressure fuel tanks. It ensures cabin comfort and safety.
Step-by-step explanation:
The APU bleed air is typically used to feed into the aircraft's environmental control system (ECS), which is responsible for regulating the cabin temperature and pressure. Additionally, the bleed air from the Auxiliary Power Unit (APU) can be utilized for engine starting, anti-ice systems, and pressurizing fuel tanks. This system is crucial for maintaining comfort and safety onboard the aircraft, especially when the main engines are not running, such as during aircraft boarding or while the aircraft is parked at the gate.