What do the internal and external air circulation modes in a car mean?

[JosieLee]

Internal and external air circulation modes in a car represent: 1. Internal circulation means the air inside the car is heated or cooled and then recirculated back into the car; 2. External circulation means the air outside the car is heated or cooled and then brought into the car, allowing gas exchange between the inside and outside of the car. The internal circulation mode closes the airflow passage between the inside and outside of the car, forming an internal airflow circulation, which can effectively and promptly prevent external dust and harmful gases from entering the car and maintain the internal temperature. The external circulation mode uses a fan to draw outside air into the car, and even if the fan is not turned on, airflow will still be drawn into the car while driving, replenishing the car with fresh air.