Canon part name is dust cover. When you print using legal sized paper, the tray must be extended via a blue button labeled LGL just inside the tray. This will cause the paper tray to stick out. So after extending the tray and loading legal stock, you then can install the dust cover before sliding the tray back in. Arrows on the cover point toward the printer. Two pegs on the cover go in the two holes on the end of the tray. Basically sits on top while you slide the tray back in.
