Every product identifier your catalog carries, and what each one is actually unique across.
Most identifier confusion comes from one question nobody asks out loud: unique across what? A GTIN is unique across the world. An MPN is unique inside one manufacturer. A SKU is unique inside one company. Mixing them in a single column is how catalogs break.
Globally unique
GTIN, and its UPC, EAN and ISBN encodings.
Manufacturer unique
MPN, unique only within one brand's own numbering.
Company unique
SKU, and marketplace identifiers such as ASIN.
The reference table
Seven identifiers, three scopes of uniqueness.
GTIN is the umbrella. UPC-A, EAN-13, EAN-8, ITF-14 and ISBN are ways of issuing or encoding a GTIN, which is why a thirteen-digit EAN and a GTIN-13 are the same number. MPN, SKU and ASIN are not GTINs at all and are not interchangeable with one, no matter how often a supplier file puts them in the same column.
The practical consequence is which field a channel will accept. Marketplaces and shopping feeds generally want a GTIN, will fall back to brand plus MPN when a GTIN genuinely does not exist, and will never accept your internal SKU as a product identity.
| Identifier | Issued by | Unique across |
|---|---|---|
| GTIN | GS1, from a licensed company prefix | All trade items worldwide |
| UPC-A | GS1 US and GS1 Canada, as a 12-digit GTIN | All trade items worldwide |
| EAN-13 | Any GS1 Member Organisation, as a 13-digit GTIN | All trade items worldwide |
| ISBN | National ISBN agencies, encoded as a GTIN-13 | All published books worldwide |
| MPN | The manufacturer, using its own scheme | One manufacturer's own catalog |
| SKU | Whoever stocks the item | One company's own systems |
| ASIN | Amazon, on listing creation | One Amazon marketplace |
GTIN and UPC
A UPC is a GTIN. A GTIN is not always a UPC.
A UPC-A barcode encodes a twelve-digit number, which is a GTIN-12. When that item is listed in a market that uses EAN-13, the same number is expressed as a GTIN-13 by adding a leading zero. Nothing about the product changed, so a catalog that stores the twelve-digit and thirteen-digit forms as different records has created a duplicate.
This is the single most common identifier bug in a distributor catalog: the same item arrives from two suppliers, one sending the twelve-digit form and one the thirteen-digit form, and deduplication fails because the strings differ. Normalizing every identifier to the fourteen-digit form before comparison removes the whole class of error.
- UPC-A is GTIN-12, EAN-13 is GTIN-13, EAN-8 is GTIN-8
- A leading zero converts a GTIN-12 to a GTIN-13
- Compare on the normalized form, not the string the supplier sent
- Keep the original supplier string as evidence of what arrived
When there is no GTIN
Brand plus MPN is the accepted fallback, and it has to be exact.
Plenty of legitimate items have no GTIN: made-to-order industrial parts, private-label goods, spare components sold from a manufacturer's own part list. Shopping feeds and marketplaces handle this with a brand and manufacturer part number pair, on the assumption that the pair is globally distinguishing even though the MPN alone is not.
That assumption only holds if the MPN is written exactly as the manufacturer writes it. Stripping hyphens, upper-casing, trimming a leading zero or splitting a suffix into a separate field turns a valid pair into an unmatched one. Store the manufacturer's own string verbatim, and keep any cleaned variant in a separate field.
- Brand plus MPN is the standard fallback when no GTIN exists
- MPN must be stored exactly as the manufacturer publishes it
- Never place a SKU in an MPN field to satisfy a validator
- Record explicitly that an item has no GTIN rather than leaving it blank
SKU and ASIN
Neither of these is a product identity you can share.
A SKU is a stock keeping unit, a code chosen by whoever holds the stock. Two companies selling the same product will have different SKUs, and one company can legitimately give the same product two SKUs in two warehouses. It is an inventory key, not a product key, and it should never be sent to a channel as a product identifier.
An ASIN is assigned by Amazon when a listing is created and is meaningful only inside that marketplace. It is useful to store as a channel reference on the product record, in the same way you store a Shopify product id, but it does not identify the product anywhere else.
- SKU is internal and can differ between warehouses or business units
- ASIN is a marketplace listing reference, not a product identity
- Store channel identifiers as channel references on the record
- Keep exactly one field for the globally unique identifier
Frequently asked questions
Clear answers before you evaluate a PIM.
Is EAN just the European version of UPC?+
In practice yes, historically. Both encode a GTIN. EAN-13 carries a thirteen-digit GTIN and UPC-A carries a twelve-digit GTIN, and a twelve-digit value becomes a valid thirteen-digit value by adding a leading zero. Modern retail scanners handle both.
Can I use my SKU as a GTIN if I do not have one?+
No. A GTIN has to come from a company prefix licensed by GS1 and has a valid check digit. A channel that validates identifiers will reject an invented value, and a channel that does not validate will silently pollute the catalog for every trading partner downstream.
What should the identifier fields look like in a PIM?+
One normalized GTIN field, one brand field, one MPN field holding the manufacturer's exact string, one internal SKU field, and separate channel reference fields for ASIN, Shopify id and similar. The supplier's original values stay attached as source observations so a disputed value can be traced.
How do I find the duplicates that identifier confusion has already created?+
Normalize every identifier to the fourteen-digit form, then group on it. Products that share a normalized GTIN but sit under different records are duplicates. Products that share a brand and MPN pair but no GTIN are the second group to review.
Continue your product data research
GTIN formats, digit lengths, and how to store them without breaking them.
Read more →MPN referenceManufacturer part numbers: the field distributors get wrong most often.
Read more →SKU designHow to design a SKU numbering system you will not have to replace in two years.
Read more →Product data qualityFind the catalog problems that block a confident purchase.
Read more →Test Nexodo with a supplier file your team already uses.
Tell us how your supplier files arrive and what your channels reject, and we will tell you whether this workflow fits.
- A person replies, normally within one business day
- Use a representative product sample
- We will say plainly if Nexodo is the wrong fit