If you are looking for a specific ID number from a torrent or Usenet release (e.g., a file named Les.Choristes.2004.iNTERNAL.1080p ), the "i---" likely refers to the internal tagging of that release group. In that case, you need a subtitle file that matches the fps (frames per second) of that specific rip—usually 23.976 or 25.000.
Les Choristes (2004), a particularly useful feature regarding subtitles is using interactive French subtitles i--- Les Choristes Subtitles