How is Meeting Attendees calculated in Insights?

Meeting attendees is defined as "total attendees in meetings" in Insights - does that mean it counts only unique attendees or does it count each attendee who attends each meeting even if the attendee has been counted from previous meetings? 

This is a question that came up during a presentation of usage for a ClearSlide account. It would be most helpful to state clearly how meeting attendees are counted when hovering over the column title or provide an article with this information.