Hi,
If you have two action model ie. if you are receiving a document from partner and sending back the response to trading partner, so you will send the original document received from partner to backend systems through model actions and then wait for response from backend systems to come, so that you can correlate the response document with the already running instance.
Here you will need the correlation service where you will constitute the logic so that the same document will be linked to already running instance, as a result the response document will be processed and sent to trading partner.
Hope it helps, let me know if you need more clarifications.
cheers!
nD
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