How are meetings categorized as Internal or External?

Worklytics categorizes meetings in one of 3 ways: 

Meetings are classified as 'Within Group' if all attendees belong to the relevant group.

Meetings with any attendee who is a member of this organization, but who is not a member of the current group, are classified as 'Other Groups'.

Meetings with any attendee who is not a member of this organization are classified as being with 'External Organizations'.