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 21 other vehicles, including the 2017 BMW 550i GT xDrive, 2017 BMW 535i GT xDrive, 2017 BMW 535i GT, 2016 BMW 550i GT xDrive, and 2016 BMW 535i GT 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.