The SharePoint Online Adapter/Remote Event Receiver is needed to get information about changes on SharePoint Elements being watched.
RemoteEventReceiver
The RemoteEventReceiver receives Events from SharePoint and prepares them for further usage. Then, it sends the prepared Events to the FireStart server. The RemoteEventReceiver can either be hosted locally on the FireStart server or, for example, on Azure. Azure supports both SharePoint Online and SharePoint On-Premise.
SharePoint
The RemoteEventReceiver will be registered on every list in SharePoint. If there is an Event fired, for example, a list entry has been created, SharePoint sends the event to the RemoteEventReceiver.
FireStart Server
The SharePoint Online adapter is integrated into the FireStart Server. If there is a new mapping created, the RemoteEventReceiver is registered to the SharePoint list.
When the FireStart Server receives an Event from the RemoteEventReceiver, the Server checks whether the Event should further be processed or not.
