How it differs from an MPN
An MPN is the manufacturer's own part number and is unique only within that manufacturer's catalogue. A GTIN is unique globally, across brands, because it is allocated through GS1. Two manufacturers can use the same MPN for different products; they cannot share a GTIN.
Why it matters for AI answers
When a model meets the same product on a maker's page, a retailer's listing and a review, the GTIN is the one field that says, without interpretation, that all three are the same item. Without it the model matches by name — and names drift: a colour in the title, last season's capacity, a bundle.
Example
A 45-litre backpack published by its maker with GTIN 00012345678905 and by a retailer with the same GTIN is one product. A retailer listing without the GTIN and with "48 L" in the title is, to a model, possibly a different one.