GTIN, UPC, EAN and ASIN
How product identifiers relate, why check digits matter, and what to do when there is no identifier at all.
GTIN is the umbrella. A twelve-digit UPC and a thirteen-digit EAN are both GTINs, normalised by zero-padding to fourteen digits. An ASIN is not a GTIN — it is one marketplace's internal identifier and does not travel.
Every GTIN carries a check digit. Verifying it costs nothing and catches transcription errors before they enter a catalogue. An identifier that fails its own checksum is worse than no identifier, because it looks authoritative.
Whole categories — apparel especially — carry no reliable identifier at all. There, identity has to be reconstructed from brand, normalised title and attributes, and the honest thing to do is publish how confident that reconstruction is.
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