All purpose, two terminal, standard. Accurately timed to combine starts with long bulb life. For use where lights are turned on more than five times daily. Also, in devices where bulb life expectancy is two years or less. No.BP45FS2: is used for 18" long 15 watt bulbs or 24" long 20 watt bulbs. No.BP45-FS4 is used for 36" long 30 watt bulbs and 48" long 40 watt bulbs.