My favourite genre of paperwork is the declaration a supplier wrote five years ago and has been photocopying ever since.
The downstream part nobody mentions: a box clears fine through the wrong door, nothing happens for months, then a verification lands and the correction is yours. They sent a template. You keyed it.
So make the supplier state in writing which door their statement is for, before the box moves. If they will not, you can certify it yourself, and then proving the content is your problem too.
From Izmir I sell knitwear into a customs union that is not a free trade agreement, and the two get confused roughly every week. One paper says the goods are in free circulation inside the union. Another says where the yarn actually came from. They prove different things, and a buyer who files the wrong one has a container in a yard while somebody re-reads the file.
From the far end, your supplier is probably not choosing between two doors. He is repeating the one he was taught, because nobody asked otherwise. At a fair in Dusseldorf a buyer told me his broker had switched lanes and assumed my paperwork would follow. It did not. Nothing had changed on my side, because nothing had been asked.
Asking in writing is right, but brace for what comes back: the same statement, retyped. The door is not the supplier's preference. It is a fact about where the content in the box came from, and finding that out means going back to the mills. Mine need a week. Certifying it yourself does not skip that week. It moves it onto you, later. Ask about the content, not about the statement.
Fair. Next entry I ask where the content came from, not which paper they like. Nothing back, no 35.