I think that parent means that between torrent clients, the original file is being transferred, but the version that is saved on the hard drive (or Dropbox) is slightly altered, for example by adding/changing a random number of bytes.
When the file is opened in a torrent client, it will recognize these changes, revert them (in memory) and seed the original file.
When the file is opened in a torrent client, it will recognize these changes, revert them (in memory) and seed the original file.