The control numbers are maintained in the WmEDIforTN webpage. So whenever you introduce a new trading partner for EDI dodcument exchange, the system looks in the EDI TN webpage and creates a new control number sequence if it does not exist and starts using them. The EDITPA control number is not of much use in most cases as some of the services that webMethods uses like ‘EDI Batching’ make use of what is stored in WmEDIforTN webpage and not from EDITPA control field.
Also, another point to note is : EDI Batching uses partners internal ID as key values to store the control number. However, if you notice the recommendation within WmEDIforTN webpage, it suggests using External ID as key values. And getControlNumber service takes in External ID to retrieve the control numbers. It is also easier to maintain from the user point of view as opposed to using the TN Partners Internal ID. We raised these issues with webMethods and they have agreed there is a mix up in their logic. Hopefully it will get fixed soon so that all services uses only External ID in the WmEDIforTN Control Number webpage.
Hope it helps