There is a check box in the Properties of the Calendar collection, on the Members tab which is called "This collection includes resource mailboxes". This check box is selected by default when you create a collection and when checked, Calendars for selected members will be synchronized two-ways. When this check box is unchecked and this change is committed, Calendars of the collection members will be synchronized one-way only. What if, for example, a collection of 500 mailboxes has only 250 switched mailboxes; is the tool smart enough to know the mailbox is not switched and sync Target to Source for 250 and Source to Target for the other 250, then change on the fly as more mailboxes switch?
Yes, the Calendar Synchronization Agent (CSA) decides which way to sync on a per mailbox basis. Whenever the mailbox is switched, the CSA will know to change the sync direction for the calendar. It does not depend on other mailboxes in the same collection.
Not Switched mailbox when Calendar collection is configured One Way = Calendar sync direction is Source to Target
Switched mailbox when Calendar collection is configured One Way = Calendar sync direction is Target to Source