If you don't ever add more than one SO to an invoice, your line UDF should be fine... just add it to the invoice work table, and update the report.

If you do invoice more than one SO at a time you'd need an invoice detail script (as referenced in the linked thread), so the line UDF value comes from the right SO.  After that, add your field to the work table, then update the report.

For testing purposes I had to use another UDF other than the one mentioned above.  I created the UDF_CoreOrderNumber on the SO Invoice details, so it shows next to each line with the different values because each line is from a different SO.  When I add the  UDF_CoreOrderNumber field to the crystal report it only pulls the "master SO" UDF_CoreOrderNumber value.  I am not understanding how a script on the details will work, and why the field isn't treated like the itemcode or any other fields on the detail level.

In the invoice line script, open an object using the AppliedSalesOrderNo (not sure if there is a Child Handle available), get the value from that object, and write it back to the invoice line.

You can pop-up a list of GetChildHandle options for a business object with something like this:

oSession.AsObject(oSession.UI).MessageBox "", "" & oBusObj.GetDataSources()

If it's not there, use a new service object (requires the user to have access to SO Inquiry). 

If there, something like this should work for the object creation.

retVal = oBusObj.ReadAdditional("AppliedSalesOrderNo")
Set oAppliedSO = oSession.AsObject(oBusObj.GetChildHandle("AppliedSalesOrderNo"))

Once the udf is written back to the invoice line on the SO Invoice data entry panel, how do you get it to show on the crystal report and not revert back to the "master" SO udf value?

Source a work table UDF from your invoice line UDF.  (Tab 3, if you want to update an existing work table UDF).

Can you see anything from these screen shots that look off?

invprint.pdfsoinvdata.pdfcrystal.pdf

Open Custom Office, UDF maintenance... SO Invoice Work... Edit fields... and open your UDF. 

Fix the data sources on tab 3... choosing your line UDF as the column.

Open Custom Office, UDF maintenance... SO Invoice Work... Edit fields... and open your UDF. 

Fix the data sources on tab 3... choosing your line UDF as the column.

Well, I am not sure if what I did was correct because I didn't use a script to get the values on the SO Inv details.  And I was completely lost on the logic of the script you were suggesting.  I basically sourced from my SOHeader for my udf on the SOInv detail, then used your data source fix on the work table(that is what was keeping me held back).

It could be you are using a version of Sage that includes the planned data flow fix (mentioned in your original post's link).