I don't see this question as Tesla trivia related so my guess is that you're really just looking for an answer. If so, I think that's where you put the carpet mat anchors if you use them so the area is reinforced. The front mats come with the anchors, but not the back, frunk or trunk, at least that's how the Tesla ones are for my S. You likely won't need them since they fit snug but I use the anchors for the driver's mat. Weathertech makes them and rebrands them Tesla and the odd thing was that Tesla sells them cheaper than the actual weathertech version, at least when I bought mine a few years ago.
If it really a Tesla related trivia test, and I got it wrong, please tell us the answer.