Total pressure = static + velocity (dynamic) pressure: the total energy the fan adds to the air.
Static pressure is the portion available to overcome resistance — ducts, filters, heatsink fins. Once air leaves the system with velocity, that energy is spent.
What this means in practice:
Choose a fan whose curve puts your operating point in its efficient/stable zone and leaves margin for resistance growth — a dust-loaded filter can double its pressure drop over the service interval.A high exit velocity is wasted dynamic pressure. A well-shaped discharge recovers part of it as static.
The classic mistake is comparing fans by free-air CFM alone. Two fans with identical free-air numbers can behave completely differently once you impose 0.2–0.3 in. H₂O (~50–75 Pa) of resistance — the one with the steeper pressure curve is the one that keeps moving air