What is an ICS or iCal URL?
Calendar programs allow you to export an iCalendar (. ics or iCal) feed URL. In a standard format, this feed URL contains event data (like times, dates, location, title, description, and time zone). Also, each event has a unique identifier or "UID."
How to set up calendar Sync with listing booking?
If your listing has any iCal feed imported, and if it does, it imports all dates from those feeds to create a reservation in the bookings table. It also happens immediately if you add a feed, and if you delete the feed, it removes all reservations added from that feed. Those reservations are not visible in your Bookings form on your listing, and they are just stored in the database to block unavailable dates for your listing and your calendar.
Please, refer to this article: how to avoid double bookings.
How do I find the iCal URL?
To find the iCal URL for a particular calendar, click the "Export Calendar" button. On the menu which pops open, select "iCal Feed," then on the "Calendar Feeds" window, click "Get Standard iCal URL."
How do I find my Google, iCloud or Outlook iCal URL?