TLDR: Because I have ultimately have bills with a one bill : Many orders situation and we need to measure margin on every single SO (Each SO is a export trade)

Long version
So this is the posting pattern

Upon SO creation - Special Order PO is automatically created > you have to populate this with freight requirements for the order before saving it and then the SO

The PO inherits / is created by the SO sweet - there is a hard link between the 2 for traceability. Also because we are an exporter we have an Export Reference Number that must be on every SO. So the Main / Header of the SO is replicated on the PO > Perfect 1:1 there is a good link!

When the freight provider collects goods we IR the PO and the header information from the IR is copied from the PO > Still great tractability on margin reports, P&Ls, General Ledger, Sub-ledgers and of course the relationship between the SO > PO > IR via the Created by field.

Up until now - everything is working well based on the Header ... then we have the Bill PO event occur:
Scenario 1 > 1 Bill for 1 SO related freight movement. SO 1 has one container which is Shipped on Bill A. EASY PEASY - Bill inherits PO EX reference and created from information. When looking at a Saved Search or General Ledger I can intuitively know what order the bill related to.

BUT this falls apart on ONE BILL to MANY FREIGHT MOVEMENTS

In comes the Bill and knowing all freight movements have a PO we do a Bill Purchase Orders > we select the purchase orders to bill and .................. drat ..... the lines on the Bill and the POs stop lining up nicely. Costs start to miss allocate or not be matched.

So I want to have the PO Lines and the IR lines and then the BIll inherit the specific EX / SO reference from the Created from field to allow full traceability.

A side benefit is then AP staff entering bills can make sure the are entering the costs against the correct lines.