The background artwork file on editable products, is an OPTIONAL artwork field, which has no purpose other than to simply create the live preview for end users, instead of the artwork file.
This is done in the case where the artwork file that you, as the printer, want to receive, has logos or other background stuff missing (say for example if you are printing on preprinted stock).
So if you put in a Background PDF, it is THAT file that the end users will see on their preview. The main artwork file is the file that will come through with an order as the print ready document.
In most normal cases you can ignore the Background Artwork Field.
If you upload a background artwork PDF. make sure it is the same size as the main artwork PDF.