Correct. Plex gathers/consolidates metadata from multliple sources, tmdb, tvdb, imdb etc… ranks its on their back-end and then that becomes the metadata for a item be it a movie, show or episode.
You can then reorder the episodes in the case of shows to match how tmdb or tvdb order the episodes providing tmdb has external id links to the corresponding episode on tvdb but the metadata is the same, its only the ordering you control. This is why shows like Firefly work when reordering between tmdb, tvdb (aired) and tvdb (dvd).
Most folks don’t understand this and think when they set plex to use tvdb ordering, its using tvdb metadata. It may or may not depending on how plex ranked the metadata beforehand but more often than not some of the data actually comes from imdb (see this thread as an example Stop scrapping IMDB Storyline Summaries for Plex Metadata).
