The slide here shows how the performance, or PQ, curve in a catalog relates to the operation of a fan. The max static pressure extreme on the top left describes a fan blowing into a closed enclosure, which is the highest level of resistance to airflow. The other extreme point on the curve, max airflow, describes a fan blowing into no obstruction at all, a condition called “free air”. Realistically, these 2 extreme points listed in the catalog are never experienced in a chassis. There is always some level of airflow resistance due to components placed in the chassis, but there will still be airflow exhausting through the enclosure.