Answer:   The document gateway service adds “hints” to TN_parms that Trading Networks uses when performing document recognition for a flat file document.
When new flat file comes to TN has to be recognized and processed document type matching. For finish this activity flat file doesn't contain metadata like XML file. It’s our responsibility to provide Meta data like who is the sender, receiver, document type. we are using gateway service to provide all this information to the TN as TN_parms pipeline variable which will be provided by gateway service. It is our responsibility to invoke or provide the gateway service to the each and every new flat file(document type). once you have been defined the Gateway service to specific flat file document make sure that you need to give the gateway service to the whoever is interested to send this type of flat file to TN in order to process the file in TN.