Mass processing refers to bundling IDocs into packets that are then dispatched and processing in the receiving SAP system. Only one RFC call is needed to transfer several IDocs. Performance is considerably better if you dispatch optimal packet sizes.
To set mass processing parameters, in ALE Customizing choose: Modeling and Implementing Business Processes ® Partner Profiles ® Generate Partner Profiles. For a given message type you can define the outbound parameters, packet size and processing mode.
If the processing mode is set to Collect IDocs and transfer, outbound IDocs of the same message type and receiver are sent in a scheduled background job in an appropriately sized IDoc packet. The IDocs can be dispatched in a scheduled background job or in ALE Administration.
Some distribution scenarios cannot support mass processing of inbound IDoc packets. This is especially true if the application sending the IDocs uses the ABAP command CALL TRANSACTION USING. In this case the outbound parameter PACKETSIZE must be set to "1".
You can find a list of function modules for mass processing in ALE Development by choosing IDocs ® Inbound processing ® Function module ® Define attributes (BD51). These function modules have input type 0.