51 12... Protective film for preventing paint abrasion in the area of the invisible joint (side frame)
WARNING: This page does not describe the selected car, but rather 50 other vehicles, including the 2016 BMW M5, 2016 BMW ActiveHybrid 5, 2016 BMW 550i xDrive, 2016 BMW 550i, and 2016 BMW 535i xDrive. However, it is still accessible from the selected car via links, so may be relevant.
To prevent paint abrasion between rear bumper trim (3) and side frame (1), protective film (2) and (4) should be glued in the area of the invisible joint.
Protective film (2) is a shaped part, protective film (4) is a foil strip (sourcing reference BMW Parts Department).
Installation note:
Protective film must not protrude over edge (5) of side frame (1) when being glued.