This is the longer rear door threshold strip for the tailgate versions of T5, T5.1 and T6.
Most standard vans come with a shorter versions of this so you can see where your floor ends at the back of the van. This will finish off the flooring neatly when doing your camper or kombi conversion. Its the small details like this that really finish off the overall look.
This trim is a genuine VW part, not a cheap copy and comes in a textured black finish with the inner foam insert.