It's part of BEP-47 [1], an extension meant to make it easier to handle single file downloads and multiple torrents serving the same file. BEP-47 aware clients

It's part of BEP-47 [1], an extension meant to make it easier to handle single file downloads and multiple torrents serving the same file. BEP-47 aware clients, which is most of them these days, I think, hide these files from the user. [1] https://www.bittorrent.org/beps/bep_0047.html https://www.bittorrent.org/beps/bep_0047.html