Intrastat Processing - Setup

Hi

I am looking at setting up the Intrastat Process and I am using the UDC 74|NT Nature of Transaction as the driver rather than SRP UDC or a custom UDC.  Now when I run the Intrstat Generation Report for Sales and Purchasing both of these reports produce no data.

There seems to be a distinct lack of documents on this process of how it all links in and ties together.  How does the system use the UDC to determine for each Sales order or Purchase order the nature of the transaction as obviously this udc is not on either of these orders etc.