A top basket within a softener, will act as a diffuser to spread the flow of incoming water evenly across the diameter of the resin as opposed to allowing the solid water stream to erode a deep divot into the resin.
Because iron reduction systems oxidize ferrous iron to cause it to be converted to ferric iron, the resulting ferric solids to be backwashed from the heavy filtration media, will often clog an upper basket screen, so an upper basket is usually not utilized. To diffuse the incoming water flow above the filtration media, an umbrella style diffuser is recommended.