The Intermediate Document (IDoc) is a container for exchanging data between R / 3, R / 2 and external systems.

SAP Application Link Enabling (ALE) uses IDocs to communicate data between logical systems.
External systems can use IDocs as the standard interface for data transfer.

IDocs are generated methods in the distribution of message types and (object).
The message type is the format in which the data for a specific business process is transmitted electronically.