Extra processing is typically processing which logically belongs in the batch program, but doesn't fit in the context of any one chunk. As this is executed in a single transaction by the chunker the potential size of any processing needs to be considered carefully. This is implemented in the doExtraProcessing method that gets the BatchProcessStreamKey and batchProcessParameters as parameters and is expected to return a BatchProcessingResult instance containing the encoded results of it's processing, see Encoding Batch Summary Information.