Valid values range from 0 - 7. Lower values will sharpen shadows (not the resolution), making vegetation more "vibrant." It gives a subtle increase in performance, but also gives more pixelated and striping effects to shadows. Higher values will make shadows softer and more blurred. Consider a value of zero if using ENB.
Any value LESS than 3 causes massive artifacting in shadows. Even 3 has some, but it is slight, and tolerable (to me). ENB suggests 3-5. Higher values cause significant performance reduction though. So the lowest value you can handle should be used.