The SP series are specifically designed to be used with heatsinks and static pressure. The Airflow series is geared more toward general case cooling as the fans require a less restrictive environment to operate at maximum performance. The fans can be mounted in the rear or at the top of the case as an intake or exhaust fan. The Static Pressure series meanwhile is designed for use in areas with a higher restriction on the airflow; a good example of this would be pushing air through the hard drive cage or processor heatsink.