The Door Mirror Casing Cover is a plastic housing/cover for the door mirror.
Attached around the door mirror unit, just in front of the leading edge of the door window, its function is to protect the mirror unit, but it also has an aesthetic and aerodynamic purpose.
The covers are, most commonly, black plastic but may be matched to the car body colour.
As the Door Wing Mirror protrudes from the side of the vehicle it is prone to collision with oncoming vehicles and although the Door Mirror unit is retractable, the cover is liable to breakage. This is the most common reason for replacement.