Has anyone ever tried creating a custom transaction line field for Journal Entries which links to PO transactions specifically? I have set a type of list/record, list/record = Transaction and I set up the filter for Type = Purchase Order. But I am at a loss in terms of what to use for the source fields. Most examples I have seen online suggest using Entity, but that doesn't make sense for a Journal Entry

What is the criteria for what PO is linked to which JE?

The use case is basically vendor bill/item fulfillment reconciliation. Linking JE to POs allows us to keep track of the link between the Vendor Bill/IF and the adjusting JE (for when the VB doesn't match the original PO).

There is a link between the vendor bill variance JE and the PO or Bill. If you use applying to transaction join in a saved search you can see what JE applies to which VB If this is the case then most likely you could get the same info from the JE as there is a link stored somewhere Try making a saved search of these journal entries and use the applied to join to see what record they apply to then you could use that info to source your custom field with a workflow