The Goodman PCP101-103 is a Downflow Plenum that is used in conjunction with the PCCP roof curb. It attaches to the horizontal return and supply openings of the unit and converts the airflow to a downflow application. The plenum sits on top of the roof curb. This accessory is not required for standard operation if you already have ducting to connect to the unit. Goodman has become one of the largest manufacturers of residential and light commercial air conditioning, heating, and indoor air quality products and systems by focusing on just one thing, building the most reliable and refreshingly affordable indoor comfort products in the market.