If the box cannot satisfy airflow demands due to not having enough available airflow (or the airflow sensor object is invalid), then open loop operation begins. In open loop operation, damper position is controlled without using the measured air volume flow value from the sensor; instead, damper position = airflow setpoint in % of nominal airflow.
When the air volume flow setpoint falls below 5% of nominal or the measured air volume flow value rises above 7% of nominal, closed loop operation resumes.