The sweep space is the gap between the bottom of the rail and the deck so that the deck can be easily cleaned off. We do not see debris accumulate under the deck when it's left open as in this picture. It does make it easier for animals to get under but if you close if off with solid skirting they will dig under and now it's a cozy home for them. With it open they may go under but the wind still blows so they probably won't stay.