This is a large plastic pan that goes underneath an air handler or furnace installed in an attic. Its purpose is home safety: in the event your equipment should leak water due to a malfunctioning of the drain system, the water will be caught in this pan and drained away through a drain hose (pan has female connection for drain fitting to attach to). This will keep the water from draining onto the ceiling below. Some cities require by code that this secondary drain pan be installed for all attic air conditioning, air handler or evaporator coil installations. Be on the safe side, and install a drain pan with a safety float switch in your new attic system.