By the strictest definition, a refugium is a 'refuge' for things to produce without being eaten by fish/inverts in your display. So, for example, growing pods for a mandarin.
Many times, we use the term 'fuge' to mean algae scrubber. Growing algae in a fuge helps grow the pods, and it does help some with reducing N&P in the system, but it depends on the volume of the refugium relative to the volume of the display.
So... it kind of depends on what your goals are, and the size of the system/fuge. Hope that helps.