You can choose between the following sharing options:
Everyone that is logged in with Office 365
Only users that have permissions to view the calendar in Outlook are allowed to see it.
This requires the users to login with their Office 365 account.Everyone that is logged in with Confluence
Only users with Confluence login are allowed to see it.Everyone with access to this page
All users that can see the page are able to see the calendar.
In a public Confluence this might be anyone on the internet.