Lastly, I don’t understand the significance of “symbol” for the calendars.
The symbol will be added to the beginning of every event for that configured calendar. For example, I have a calendar of movie release dates merged in with my social calendar. I keep the “calendar-check-o” for the social calendar, but I use “video-camera” for the movie release calendar.
The end result is that every entry that is a movie release shows a video camera icon and my social calendar shows a calendar with a check icon.