In Outlook 2007 and later, all Private items will show with a lock icon in the Calendar:
In Outlook 2003 and previous, all Private items will show with a key icon in the Calendar: In the Tags group on the Appointment or Meeting tab, click the “Private” button with the lock icon. Outlook 2010, Outlook 2013 and Outlook 2016.In the Options group on the Appointment or Meeting tab, click the “Private” button with the lock icon. To do this, double click on the appointment or meeting to open it in its own window and choose: If you only want to block certain individual appointments from showing their contents when someone schedules a meeting with you or opens your Calendar folder, then you can mark an appointment as private. To prevent co-workers from seeing details via Free/Busy times, you can either publish your appointment as Private or you can change the permissions on your Calendar folder. When co-workers plan a meeting and are looking at free/busy times via the Schedule Assistant, they can see the details of any appointment if they hover the mouse pointer over the appointment.
