From MINIUSA Aftersales:

This accessory protects the paint of the rear bumper from getting scratched by loading and unloading the MINI. With adhesive backing, the bumper protection can be easily installed on all MINI hardtop rear bumpers, including the aero kit bumper. The protection stripe matches the structure of the other black edges around the bumper and the fender flares.

In case you’re wondering, the part number is 82 11 0 393 995 and it retails for $40.