Looking at your diagram, it would represent the solid sheet metal surrounding the fins. Typically, if you blow into the heatsink, you don't want the sides covered by sheet metal. However, try flipping the fans with it in place and see if your temps improve. Keep in mind that looking at the diagram, the exhaust air will exit the assembly through the small rectangular opening. Whatever components lie just beyond that opening will get heated by the exhaust plume. This does not happen with the shroud removed. Look at some dealer Web pages that have pictures of different heatsinks. You will see some with no sheet metal around the fins, like the GlobalWin FOP and WBK models. Those are blower types. Some will have a shroud around the part of the fins near the mounting point of the fan. They are typically sucking types.
Hoot