PS: From what I understand, a sump is just a place to hide all the equipment so that everything is out of the main display tank.
Refugium provides a safe place for creatures to reproduce (copepods, amphipods) that would normally be preyed on. Also where nutrient export via macroalgae, sponges, filterfeeders, ect.
That said, sometimes people combine the duties of a refugium and a sump into one by sectioning off different sections for it.