Looks like the rear window side spoilers/ wind deflectors. These run vertically down the far left and far right sides of the rear glass. You dont have to have them.
From the shape, I thought these parts were the side spoilers, but the Aerodynamic spoilers don't come with them. It looks like
@dronners is right as it is the shape of the part of the spoiler that goes onto the rear window.
This is why I'm after genuine (clear!) instructions, to see how a SEAT suggest it goes on. Things like, it looks like the instructions say that the sealant just goes on at each end, but when you watch some YouTube vids, they put the sealant all over the area within the double-sided tape.