This might be a simple question for some of you but I don’t know what a "auto regulator" ballast does. I was on graybar looking at some GE street lights and noticed that some have this auto regulator ballast and some have a NPF ballast. What is the difference between a NPF ballast and a auto regulator ballast? Also what one is better? Thank You for helping! 

My guess would be an autotransformer (probably CWA) ballast, as is quite common in North America. Just a thought...

CWA Indeed. It controls the output power (rather than output current which is what other ballasts tend to do) so has better stability with line voltage fluctations

CWA Indeed. It controls the output power (rather than output current which is what other ballasts tend to do) so has better stability with line voltage fluctations

Actually CWA do control the output current only, but they do provide quite good line regulation (so compensate out line voltage fluctuations).
The name "constant wattage" came from constant power to lamp with specified, accurate arc voltage (= MV's at the time these ballast were introduced), where the only variable is the mains voltage. This is implemented by regulating the current. In the today's perspective of lamps with the arc voltage dependent on the tube temperature the term "constant wattage" become quite misleading.