What's the best way on a packing slip to get the sales order quantity, fulfillment quantity, and the outstanding quantity. Currently we loop over the source record to create a hash of quantity/quantityfulfilled, and then sub it from the shipment quantity. This breaks when an item appears twice (key=item) though since there's no unique key (that I can see) between the order line and fulfillment line.

Hey @Christopher Quackenbush, just joined the channel (hence the late comment) and you are correct. This is the native linking from IF lines to their respective SO lines. This is especially helpful when creating multiple IF records to one SO.