Sorry if this has been asked before, I wasn't able to find any clues..
I've created two events, one "open" and one "closed". The system tells me that only "invited users can see expanded event details".
For my (uninvited) test user, both events are displayed exactly the same, with all event info, when&where, attachments, etc..
I'd really like to know what the expanded event details are (I am aware of the different ways to join open/closed events)
I personally feel like the documentation description explains what a Closed Event is the best;
Closed events can be seen by anyone with access to the place where they're posted, but do not have an RSVP link for users unless they have already been invited to the event.
Hope this helps.