Here you can find detailed information about calendars and data sources that you can overlay in Virto Calendar Overlay for SharePoint.
With Virto Calendar Overlay, you can overlay various calendars in a single calendar view. Let’s see which calendars you can use as data sources.
You can create an unlimited number of calendars with multiple data sources.
Exchange Online calendars
With Virto Calendar Overlay, you can combine any number of Exchange Online shared and personal calendars in a single calendar. This is the fastest way to create a calendar overlay because no special fine-tuning is required for this data source.
If you use a shared mailbox for booking meeting rooms in SharePoint, you can add it to Virto Calendar to track meeting rooms in a single place.
This is one of the most popular options to overlay calendars. Use SharePoint lists(including lists in a calendar view) as data sources for your overlay. You can overlay an unlimited number of lists in a single view.
Public folders in SharePoint are designed for shared access. Since every public folder has a mailbox, it can be easily added to the Virto calendar as a data source.
Internet calendars (iCal feeds)
An iCalendar feed or .ics feed isa unique calendar format you can use to work with most calendar solutions. Generate iCal links and overlay events from Google Calendar or any other external calendar that supports iCalendar feeds.