When creating a new event or task, a user can specify whether the event or task is Public, Private, or Time and Date Only (confidential):
Anyone with read permission to the user’s calendar can view the event or task.
Only owners of the calendar can view the event or task.
These are confidential events and tasks. Owners of the calendar can view the event or task. Other users with read permission to the calendar can see only “Untitled Event” on the calendar, and the title is not an active link.
The calstore.filterprivateevents determines whether Calendar Server filters (recognizes) Private, and Time and Date Only (confidential) events and tasks. By default this parameter is set to "yes". If you set calstore.filterprivateevents to "no", Calendar Server treats Private and Time and Date Only events and tasks as if they are Public.