What is a SKU (stock keeping unit)? A guide for UK suppliers

A retail buyer and the supplier that sells to it can both say "SKU" about the same product and mean different numbers. Each side means its own code. An EDI order line has places for those private codes next to a shared one, the GTIN, and your system has to know which is which before it can act on the order.

What is a SKU?

SKU stands for stock keeping unit. Cambridge Dictionary defines it as "a number or set of numbers given to a product to show which particular one it is."

GS1 US, a member of the standards body behind the barcode, calls the SKU "an internal tracking metric for retailers" and says "a SKU number is internal and created by a specific brand or retailer." Its product data service lists the SKU as an "Internal product identifier of the stock keeping unit", in its own field beside the GTIN.

None of these sources sets a format for a SKU. A shop's SKU is the shop's code; your SKU is yours.

SKU, GTIN and your customer's item number: what is the difference?

Two of the three codes are private. One is shared by everyone who handles the product.

GS1 US puts the difference in one line: "Two retailers can sell the same item with the same UPC codes, but they will both use their own SKU number." GS1 US writes UPC, the GTIN-12; a UK retail unit usually carries a GTIN-13 in an EAN-13 barcode.

The GTIN is fixed in a way a SKU is not. GS1's General Specifications say "No downstream party (e.g., distributor, wholesaler, importer, merchant) may assign a different GTIN to a trade item that already has a GTIN". They also advise against building internal codes into the GTIN: "The embedding of internal codes is discouraged because it is often found that the rules for changing them differ from the rules for changing a GTIN."

The GTIN guide covers the GTIN's structure and change rules. Its answer to whether a GTIN is the same as a SKU: "A SKU is your own product code, and you can change it on your own terms."

Where does each code sit in an EDI order, despatch advice and invoice?

In GS1's EANCOM standard, a product line holds many segments, and two of them are for its product codes.

LIN, the line item. In an order, "This segment is used to identify the item being ordered." The rule is firm: "If Global Trade Item Numbers are available it is mandatory to use GTIN within the LIN segment." The GTIN is marked with code SRV.

PIA, the additional product id. GS1's note: "This segment is used to specify additional or substitutional item identification codes such as a buyer's, or seller's item number." Two codes on its list matter to a supplier: "IN = Buyer's item number" and "SA = Supplier's article number". The supplier's article number is the place for your own code, your SKU.

GS1's own worked order puts GTIN 5412345111115 in LIN and the buyer's code ABC1234 in PIA, with the note: "The GTIN 5412345111115 provided in the LIN segment refers to the internal buyer's item" number ABC1234.

The despatch advice and the invoice follow the same pattern.

The standard provides the places. Which codes a given customer actually sends, the GTIN alone or the GTIN with another code, is set in that customer's own message guide, not by GS1.

What do UK buyers ask suppliers to quote?

The GTIN is the common thread. GS1 UK's ASN implementation guideline, version 1.5 from March 2015, says the despatch advice "will also include information about the contents of each unit, using GTINs to identify each product line". The ASN guide covers the message itself.

Buyers set out in their own documents which codes they want to see.

• NHS Supply Chain. Its Operational Supplier Requirements, Version 4.1, uploaded in March 2025, list what delivery advice notes must contain as a minimum, including "MPC (Supplier Item Code)" and "NPC (NHS Supply Chain Item Code)". MPC is the manufacturer's product code; NPC is the national product code. On invoices: "Invoices should quote the same manufacturer's product code provided on our purchase order."
• Ocado. Its Supplier Manual v3.0, from September 2023, lists "Product descriptions & Ocado SKU codes" among what delivery documentation must contain. Here "SKU" means Ocado's own code, not yours.
• Travis Perkins and Wickes. Their supplier manuals (Travis Perkins, October 2021; Wickes, January 2022) score delivery line by line. As the OTIF guide puts it: "Travis Perkins and Wickes count order lines for each SKU."

Each manual binds only that buyer's suppliers. For the codes a customer puts on its EDI orders, read that customer's own message guide.

What happens when the codes do not line up?

The order line has to land on the right product in your system, and the despatch advice and invoice have to describe it the way your customer recorded it.

The EDI number guide explains the GTIN side. With a wrong or missing GTIN, orders can be "matched to the wrong product line, which then holds up picking, despatch and invoicing."

The ASN guide describes what follows a mismatch between the despatch advice and the delivery: "Downstream, the same mismatch causes invoice queries, because the invoice cannot be matched cleanly against what the retailer's systems recorded as received."

Pack level needs care. The purchase order guide quotes the GTIN guide: "The GTIN on an order line is the one for the unit being ordered, and that is often the outer case rather than the single pack." If your SKU is the single bottle and the order names the case, the link between them has to say so.

Without that link, people do the matching by hand. A Scottish drinks company's systems manager, quoted in the purchase order guide, described staff who would "print and match them to a form which showed the customer number and SKU, to then rekey this data into our own ERP system."

How do you keep SKUs, GTINs and customer item numbers in step?

GS1 names the job. Its answer on internal codes says: "Often internal codes for an item will have to be cross-referenced to the GTIN for the item."

The EDI mapping guide puts it in supplier terms: "a retailer's order names each product by its GTIN, and your map has to turn that into your own product code." It adds: "That only works if each product's GTIN is held against it in your ERP."

Five checks keep the codes in step. The checks are our advice; the rules behind them are GS1's and your customers' own.

1. Hold the GTIN against each SKU, at every pack level you sell.
2. Hold each customer's item number against your SKU where that customer sends one. It differs at each customer.
3. Record which pack level each code refers to, so an order for a case does not land on a single unit.
4. Quote back the codes the order used on the despatch advice and the invoice.
5. Check the GTIN when a SKU or pack changes. The GTIN guide sets out when GS1's rules call for a new one, and your customers need to hear about it.

One worked example comes from a UK drinks brand that had "persistent SKU failures with orders sent to a major retail partner". In the words of the case study: "A dynamic look-up table was built to automatically assign the correct SKU, warehouse code, pricing and pack size for every product sent to the affected retailer." That was one supplier's fix for one retailer's orders. The principle travels: automation can only read what the item master holds.

How Transalis runs the mapping for you

Transalis is a managed outcomes provider for B2B integration, EDI and eInvoicing. Its EDI page says: "Transalis delivers EDI as a fully managed service: we handle format translation, partner onboarding and compliance, so documents flow automatically between your business and 15,000+ trading partners." In its own heading: "We run onboarding, mapping and go-live for you".

When a code does not match, you can find it in the message. As the EDI number guide puts it, "Transalis DataTrack™ gives you visibility into individual messages, so you can see where an identifier does not match and correct it before it causes a bigger problem further down the chain."

Map your supply chain with the codes your customers already send.