IDoc
IDocs (Intermediate Documents) are standard containers for exchanging data between applications. Between SAP applications they are transferred using the ALE (Application Link Enabling) layer which again uses either tRFC or File technology as the underlying technique.
An IDoc contains different types of information. It contains the application data to be exchanged (e.g. a sales orders) as well as technical data providing information from where to where the IDoc is supposed to be sent. Furthermore, the IDoc also contains status information that shows which processing step within the data exchange the IDoc is currently in. These statuses can indicate error situations or success situations. Some of them can be intermediate statuses that are indicating backlog situations.
For an end-to-end ALE monitoring it is necessary to monitor the various IDoc statuses in the outbound as well as the inbound direction.