I inherited this with the boat but never asked the previous owner what it was. Clearing out yesterday I just couldn't see how it fits anywhere on the boat but it was obviously designed exactly this way with clips and rings and lines in specific places.
The shape might be a bit misleading at first, as we expect lee cloths, well, to have a different aspect ratio. But many (including ours) are fixed at the outboard side of the bunk, under the cushions, so that the weight of the sleeper helps to hold them in place as well as the fixings. That's certainly what it looks like to me.