This is an APC Rack Air Containment Rear Assembly, a physical airflow-management accessory rather than a standalone appliance — it's designed to fit the rear of an APC NetShelter SX 42U enclosure paired with a 600mm InRow cooling unit. Rack air containment components like this seal off the gap between server racks and in-row cooling units so that hot exhaust air from equipment is channelled directly into the cooling system's return path rather than mixing back with the cool air being supplied to equipment intakes. This significantly improves cooling efficiency in a data centre or server room, letting InRow cooling units work more effectively and reducing hot-spots. It's a compatibility-specific part (matched to the NetShelter SX 42U rack and 600mm InRow cooling unit), so it should only be purchased alongside, or as a replacement/addition for, that exact combination of equipment rather than as a general-purpose accessory.