What is a proof of origin?

A proof of origin is the document behind a claim for the lower duty rate. Under most newer agreements it's a statement the exporter or producer writes themselves. Some older agreements and schemes still use a certificate stamped by an authority.

Who writes it

Self-certification is now the norm. The exporter, the producer or under some agreements the importer states that the goods originate. Older systems use a certificate issued by an authority, like the EUR.1 movement certificate. Some ask exporters to register first, as the EU does with REX.

What you need to keep

Whoever signs it has to be able to back it up, usually for several years. That means keeping the bill of materials, the costs and the supplier declarations the claim relied on.