The instance flows to a Termination Wait activity after the Engine finishes running the flow objects that might exist between the Process Creation activity and the Termination Wait Activity. When the instance in the subprocess arrives to the End, the subprocess notifies the associated Termination Wait. Then the Engine maps the output arguments of the subprocess to the process instance variables, using the defined argument mapping.
If the instance in the parent process arrives to the Termination Wait activity before the Engine finishes running the subprocess, and the property Process Notification Immediately is selected, then the Engine processes the notification and the argument mapping in the same transaction.