A refugium is a refuge for critters and macro algae. The algae needs light to grow. Reverse daylight can help a little to stablize PH because of the photosynthesis occuring with the macroalgae. A sump is a water reservoir that holds equipment that won't fit in the tank or would look unsightly like a skimmer, pump, heater etc. There is no need for light on a sump. I hope this helps clarify it.
Mike