If the calendar is set to 'Public'  by default, users can decide whether or not to filter out their calendar to only show entries for their department, manager or site location. 

Administrator-level users have the ability to be able to define this filter for the public calendar at the highest level, rather than giving the employee the option.

To apply these settings, an administrator will need to go to:

Administration>Company>Settings>Display Settings

Under the 'Calendar' section, you can decide which restrictions to apply to the public calendar. 

If you choose to select department, site or manager the public calendar will only show employees entries for each of those choices respectively.