The “optional info” part is basically just what you’ll be using to identify files in your file explorer. Plex will not use this – not even for episodes where it cannot find metadata… in such a case, Plex will just show the items as “Episode {X}”. This is unless you have your library configured to consider embedded metadata – in which case the embedded title will be used.
Personally I try to steer clear from embedded metadata with tv-shows as it can give you some weird results (e.g. accidentally matching a show based on the file’s embedded metadata instead of file names).
It’s worth to keep in mind that the online databases used by Plex usually reserve “specials” to be actual specials (e.g. X-Mas/holiday specials…), not the classical extras you might get from a disc. As piskieee pointed out, there’s currently no good solution for those unless you stick to manually labelling them from within Plex (which can be painful and those updates will be lost if you have to rebuild your library).
There’s a 3rd party agent that aims at finding extras for shows/seasons/episodes – those will even be displayed in a few Plex clients.
And… there’s of course a feature suggestion for Plex to officially support dvd-type extras to for shows/seasons/episodes (incl. properly reflecting them in the Plex apps.
