The basis for distributing application functions is asynchronous distribution of messages via ALE outbound and inbound processing.
Message distribution is based on IDocs. IDocs are created and dispatched in distributed systems using message types and SAP business object methods (BAPIs).
As of SAP Release 4.0 synchronous and asynchronous interfaces should be implemented as BAPIs. If you want to implement a BAPI as an asynchronous interface, you have to generate a BAPI-ALE interface for an existing BAPI. SAP provides these interfaces for many BAPIs. You can also generate such interfaces yourselves.
You can implement ALE business processes with these interfaces.
For more information see the ALE Programming Guide under Distribution Using BAPIs and Distribution Using Message types.
In the outbound system an IDoc containing the data to be transferred is created and prepared for dispatch in the outbound system.
Then the IDoc is transferred to the target system.
In the target system the IDoc starts inbound processing. The data is processed and then posted in the application either fully automatically or part manually.
Inbound and outbound IDocs can be processed individually or in a packet. In mass processing several IDocs are processed together in one packet.