The external product identifier — usually UPC, EAN, GTIN or ISBN.
Barcodes follow public standards (UPC-A, EAN-13, GTIN-14, ISBN) and identify a product globally rather than internally. Google Shopping, Amazon, Meta and most marketplaces require them for listing eligibility. Unlike SKUs, barcodes are meant to be identical across every store that sells the same manufacturer product. Missing or malformed barcodes are the single most common cause of Google Merchant Center disapprovals and marketplace listing failures. Store them on the variant, not the product.
