Product identifier (GTIN)
A product identifier is a code that names your exact product in a way every system recognizes. The most important is the GTIN (Global Trade Item Number), the number behind a product's barcode, which includes UPC and EAN codes. It is not the same as your internal SKU.
How it works
A GTIN is global and shared: the same code identifies the same product on your store, on a marketplace, and in a review database. That lets an agent recognize your product as the same item it has seen elsewhere and pull in reviews and price comparisons. Your SKU, by contrast, only means something inside your own store.
A commerce example
Two listings sell the same speaker. The one with a GTIN can be matched to its reviews and price history across the web, so an agent can vouch for it. The one with only an internal SKU looks like an unknown product and is harder to trust.
What this means for your store
Fill in the GTIN on every product that has one, in your structured data and your product feed. It is one of the highest-value, lowest-effort fields you can complete.