51 12... Protective film for preventing paint abrasion in the area of the invisible joint (side frame)
WARNING: This page is about a different car, the 2011 BMW 750xi, 2011 BMW 750Lxi, 2010 BMW 750xi, and 2010 BMW 750Lxi. 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.