Out Outlook add-in needs to know if the calendar event being edited is part of a series. Currently the only way to get this requires saving the Event as a draft, then pass the ID to a backend API to call Graph to get the Event object by ID, and then look at the .Type property. Also, if the event is an "Occurrence" then the SeriesMaster records needs to get looked up to find the Recurrence Range, etc., properties. This information should be directly readable by the Add-In via the JavaScript API. Related to that the Add-In needs to know if the Edit is "Edit All", "Edit Single" or the OWA-specific "Edit here and forward".