External and internal sharing options for primary calendars are configured to share only free/busy information but hide event details
External and internal sharing options secondary calendars are configured to share only free/busy information but hide event details. However, users can change the settings for these to share full event details
This gives the ability to create additional calendars for things such as school events and share event information while the primary calendar, which is personal to the user, will always remain private
This is particularly useful for Sites as it would allow a calendar set up for a school to be embedded and anyone accessing this site can view details of events rather than just seeing that a time and date is free/busy.