Multithreading batch jobs

  while (queryProcessorRun.next())             

    {
       myTable = queryProcessorRun.getNo(1);
      MyIndivudalTaskService processIndivudalTaskService = new MyIndivudalTaskService ();
      processIndivudalTaskService.parmSalesId (myTable .OrderNum);
      processIndivudalTaskService.parmLineNum (myTable .LineNum);
      processIndivudalTaskService.parmQty2Delv(shortPicksOrderProcessorTable.ProcessedQty);

      processIndivudalTaskService.batchInfo().parmCaption(strFmt("Processing for Purch order %1", myTable .OrderNum, myTable .linenum));
      batchHeader.addTask(processIndivudalTaskService);
      batchHeader.addRuntimeTask(processIndivudalTaskService, BatchHeader::getCurrentBatchTask().RecId);
//This is where you are listing the added service to a List
      taskServiceList.addEnd(processIndivudalTaskService);           
    }
//This is where your code adds the header to a batch:
MyProcessCompletePurchTableController processCompletePurchTableController = new MyProcessCompletePurchTableController ();
    processCompletePurchTableController.parmHeaderPurchId(_orderNum);
    batchHeader.addRuntimeTask(processCompleteSalesTableController, BatchHeader::getCurrentBatchTask().RecId);

Now you need to ensure that the header waits till all the lines get executed without any error:
while (listEnu.moveNext())
    {

      MyIndivudalTaskService taskService = listEnu.current();
      batchHeader.addDependency(processCompletePurchTableController, taskService, BatchDependencyStatus::Finished);       
    }
batchHeader.save();