Event Visibility is a trip-wide setting that you can set for group trips. You can choose to set the Events - not entire itineraries - as private or public.
When you set the Events as Private this means travelers that have been added to the trip can only see the Events on the itinerary that they are tagged in or if there is an Event that has no travelers tagged to it (visible by everyone).
When you set the Events as Public this means that anyone can see the Events on the itinerary even if travelers are tagged to certain Events. This also means that the travelers tagged to the Events can be seen by others on the online itinerary or mobile app.