What type of air flow reading must the AFM Indicator provide?

The AFM Indicator must provide readings in cubic feet per minute because this unit of measurement is specifically suited for evaluating the volume of air flowing through the brake system per minute. This is crucial for ensuring that the air brake system operates effectively, as it helps gauge whether there is adequate air supply to activate the brakes properly. Understanding airflow in cubic feet per minute allows for better assessment and monitoring of the system's performance, ensuring safety and efficiency in train operations. The use of other units, such as pounds per square inch or gallons per minute, would not accurately reflect the air volume necessary for brake function.