18‑gauge steel, they are installed on the ends of the sections and are used to have a stronger attachment for the hinges.
flexible, it is installed on the top of the section for better sealing. For very wide doors, it keeps the water tightness of the door when bowing of the top section occurs (bowing is the curving of sections which can be caused by indoor and outdoor temperature differences).
made to ensure a good air circulation in the garage. 3" (76 mm) and 4" (100 mm) diameter sizes are available.