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You open Amazon Seller Central and start creating your first product listing. At first, everything seems straightforward — until you’re asked for a UPC or EAN. A moment later, there’s an ASIN. And when you look into FBA requirements, you come across something called FNSKU. This is usually the point where things stop feeling intuitive. Are these codes interchangeable? Do you actually need all of them? And why does Amazon generate its own identifiers if your product already has one?
If you’ve found yourself jumping between tabs trying to make sense of it, you’re not alone. This confusion is incredibly common, especially for sellers entering Amazon EU, where EANs appear more often than UPCs and the rules can feel slightly different from what you might know from other markets. The good news is that there is a clear logic behind all of this. Each code serves a different purpose — the problem is that Amazon doesn’t explain it in a way that’s easy to follow when you’re in the middle of setting up your listings.
In this guide, we’ll break it down step by step, so you know exactly which code is used where and what role it plays in the FBA process.
Why Amazon uses multiple product codes (and why it’s so confusing)
At first glance, it feels like Amazon is overcomplicating things. You already have a barcode for your product — so why introduce more codes on top of that? The confusion usually comes from the fact that these identifiers look similar, but they don’t serve the same purpose.
The key difference is this: some codes are global product identifiers, while others are internal Amazon identifiers. As such, they operate on different levels of the system and are used at different stages of the selling process:
- Global identifiers like UPC and EAN are used to define the product itself. They are standardized codes that exist outside of Amazon and are recognized across marketplaces, retailers, and logistics systems. In simple terms, they answer the question: what is this product?
- Amazon-specific identifiers — ASIN and FNSKU — serve a different role. They exist only within Amazon’s ecosystem and are used to manage listings, sellers, and inventory inside the platform. Instead of defining the product globally, they help Amazon answer questions like: which listing is this? and who does this inventory belong to?
Those codes also show up at different steps of the setup process. When you create a new listing, Amazon asks for a UPC or EAN to identify the product. Based on that, it assigns an ASIN to place the product in its catalog. Then, if you send inventory to FBA, you need to use FNSKU labels so Amazon can track your specific units inside its warehouses.
So Amazon isn’t duplicating the same function — it’s separating product identification, catalog management, and fulfillment into different layers. Once you see that distinction, the system starts to feel a lot more predictable.
What is UPC (and when you need it)
UPC (Universal Product Code) is basically the standard barcode used to identify a product globally. You’ve definitely seen it before — it’s the one printed on retail packaging and scanned at checkout in stores. What matters here is that UPC doesn’t belong to Amazon. It’s part of a global system (called GTIN) managed by GS1, which means the same code is used across different marketplaces, retailers, and logistics systems. In other words, it answers a very simple question: what product is this?
UPC is usually a 12-digit number displayed under a barcode on the product packaging or in your product documentation, for example it might look like this 012345678905. Meanwhile if you are manufacturing your own product, you need to obtain it from GS1. . This usually means registering your company with GS1, purchasing a set of barcodes, and then assigning a unique code to each product (or product variation). Once assigned, that code stays with the product and can be used across marketplaces, including Amazon.
On Amazon, you’ll run into UPC most often when you’re creating a new listing; if your product isn’t in the Amazon catalog yet, the platform will ask you for a UPC so it can register that product and create an ASIN for it. One important thing to keep in mind though: UPC identifies the product, not your specific offer. If other sellers are offering the same item, they’ll be using the same UPC — it’s not something unique to your account. But the plus is that if the product already exists on Amazon, you usually don’t need to enter a UPC again. Instead, you just match your offer to the existing listing (ASIN).
What is EAN (and why it matters in the EU)
EAN (European Article Number) works almost exactly the same way as UPC — it’s a global barcode used to identify a product. The main difference is where it’s most commonly used. While UPC is more typical in the US, EAN is the standard across Europe. In practice, an EAN is usually a 13-digit number shown under a barcode, for example, 4006381333931. You’ll find it on product packaging, just like a UPC. And just like UPC, it’s part of the global GTIN system managed by GS1.
From an Amazon perspective, EAN plays the same role as UPC — it’s used when you’re creating a new product listing. If your product isn’t already in the Amazon catalog, you’ll need to provide an EAN so Amazon can recognize it as a unique item and assign it an ASIN. The reason this matters more in the EU is simple: most products in European markets already use EAN instead of UPC. So if you’re entering Amazon EU, this is the format you’ll encounter more often — both when adding your own products and when matching existing listings.
Functionally, though, there’s no real difference in how Amazon treats them. Whether you use UPC or EAN, the purpose is the same: to define the product at a global level before it becomes part of Amazon’s catalog.
What is ASIN (Amazon’s internal product ID)
ASIN (Amazon Standard Identification Number) is Amazon’s own way of identifying products inside its platform and unlike UPC or EAN, it's used only across Amazon. Each ASIN is a 10-character combination of letters and numbers, for example, B08N5WRWNW. You can usually find it directly on the product page (in the URL or product details section) or in your Seller Central dashboard once the listing is created.
Amazon does this to avoid duplicate product pages for the same item. Instead of having multiple listings for one product, it keeps a single product page (one ASIN) and groups all sellers under it. This way, customers see one consolidated listing with different buying options, rather than several nearly identical pages.
Here’s how it works in practice. When you go to Seller Central and choose “Add a product”, Amazon first checks if your product already exists in its catalog. If it finds a match (based on title, brand, or barcode), it will show you an existing ASIN — and you simply select it and add your offer. If no match is found though, you'll be asked to enter details like product name, brand, and UPC or EAN. Based on that information, Amazon creates a new ASIN and assigns it to your product automatically. From that moment on, this ASIN becomes the main reference for that product on Amazon and every seller offering the same item will be connected to that same ASIN, instead of creating separate listings.
So basically, if you are selling a unique item (like brand-made cosmetics), you'll need the EAN/UPC codes to create a listing in Amazon and have it assign you a new ASIN. If you sell a product that already exists in Amazon's database though, Amazon will automatically show you the matching ASIN.
What is FNSKU
FNSKU (Fulfillment Network Stock Keeping Unit) is the code Amazon uses to track your specific inventory inside its fulfillment centers. Unlike UPC, EAN, or even ASIN, this one is tied directly to you as the seller. It’s usually a 10-character code (letters and numbers) that Amazon generates for each of your products when you create an FBA shipment. You can find it in Seller Central, and it’s typically printed as a barcode label that needs to be placed on every unit you send to Amazon. Why?
Each unit is scanned when it’s received, and the system needs to decide: which seller account should this item be assigned to? If the product only had a UPC or EAN, Amazon would know what the product is, but not who it belongs to. That’s a problem when multiple sellers are offering the same item under one ASIN. This is where FNSKU comes in. When Amazon scans the FNSKU label, it immediately links that specific unit to your seller account. That’s how your inventory is correctly registered, stored, and later picked to fulfill your orders — not someone else’s.
What you need to keep in mind is that every unit you send to FBA needs to have a scannable FNSKU label placed on the packaging, ideally on the outside, where it’s easy to scan during receiving and picking - and all other barcodes covered.During inbound, Amazon workers will scan whichever barcode is visible and if they accidentally scan the UPC/EAN instead of the FNSKU, the system won’t be able to assign that unit to your seller account. But since the requirement to cover all other labels is clearly marked in FBA guidelines, the shipment being rejected will be treated as your fault, not the Amazon's worker.
That’s why, in many cases, you need to physically cover the original UPC or EAN barcode with the FNSKU label. If both barcodes are visible, there’s a real risk that during receiving an Amazon worker scans the wrong one. Covering the original barcode removes that ambiguity as it forces every scan in the warehouse to use the FNSKU, which is the only code that links the unit directly to your seller account.
How these codes work together in practice
The easiest way to understand these codes is to follow the exact path your product takes (from creating a listing to sending inventory into FBA) and look at when each code shows up and what it is used for.
- You start by adding a product → this is where UPC or EAN comes in
In Seller Central, you go to “Add a product” and begin creating your listing. At this stage, Amazon needs to understand what the product actually is and that’s why it asks for a UPC or EAN — a global identifier that defines the product outside of Amazon. If your product isn’t in the catalog yet, this code is what allows Amazon to recognize it as a new item and move forward with listing creation. - Amazon checks the catalog → and assigns (or shows) an ASIN
Once you enter the product details and barcode, Amazon compares that information with its existing catalog. If it finds a match, it shows you an existing ASIN and you simply attach your offer to it. If no match is found, Amazon creates a new ASIN for your product. This ASIN becomes the product’s identity inside Amazon — it’s what ties together the product page, content, and all sellers offering that item. - You decide to use FBA → the process shifts to logistics
At this point, your listing is ready, but you still need to get your inventory into Amazon’s warehouses. When you create an FBA shipment, you’re no longer dealing with product identification at a catalog level — now it’s about tracking physical units. - Amazon generates FNSKU → one for each of your products
During the shipment setup, Amazon assigns an FNSKU to each product in your offer. This code is unique to your seller account and that specific product, even if other sellers are using the same ASIN. You can download these labels directly from Seller Central — they’re what you’ll need to print and apply before shipping. - You prepare and label your products → this step is physical, not just digital
Before sending your inventory, you need to make sure every unit is labeled with the correct FNSKU barcode. This usually means printing labels and placing them on each individual product or its packaging. If there’s already a UPC or EAN visible, it needs to be covered so that only the FNSKU can be scanned in the warehouse. - Amazon receives your shipment → and assigns inventory based on FNSKU
When your shipment arrives at the fulfillment center, each unit is scanned. The system uses the FNSKU to assign those units to your seller account. From that point on, all warehouse operations — storing, picking, packing, and shipping — rely on FNSKU to make sure the correct seller’s inventory is used to fulfill each order.
From this example you can see that each code answers a different question at a specific step. When you’re creating a listing, Amazon needs a UPC or EAN to decide whether the product already exists or if a new entry should be created. Once that’s done, everything in the catalog is organized under an ASIN — that’s the product page you and other sellers attach to. But when you move to FBA, the focus shifts completely. Amazon is no longer asking “what is this product?” — it’s asking “which seller does this unit belong to?”. And that’s exactly what the FNSKU is used for when your shipment is received and processed in the warehouse.
Quick comparison: UPC vs EAN vs ASIN vs FNSKU
When you’re in the middle of creating a listing or preparing an FBA shipment, these codes tend to blur together. You see different fields, different labels, and it’s not always obvious which one Amazon is asking for at that moment. This is where a quick side-by-side comparison might help - and for this we created a simple comparison table you can use when in doubt which code should you use:
| code | who creates it | what it identifies | where it’s used | required for Amazon? | can you change it? |
|---|---|---|---|---|---|
| UPC | GS1 (you obtain it) | the product (globally) | product creation (mainly US) | yes, for new listings | no |
| EAN | GS1 (you obtain it) | the product (globally) | product creation (mainly EU) | yes, for new listings | no |
| ASIN | Amazon | the product in Amazon’s catalog | listings & product pages | always (auto-assigned) | no |
| FNSKU | Amazon (per seller) | your inventory (per seller + product) | FBA labeling & warehouse tracking | yes, for FBA | can be re-generated |
If you’re unsure which code to use, it helps to look at what you’re trying to do at that moment. If you’re creating a new product, Amazon will ask for a UPC or EAN. If you’re selecting or managing a listing, you’ll be working with an ASIN. And if you’re preparing products to send into FBA, you’ll need FNSKU labels for each unit.
So… which barcode is Amazon actually asking for?
At first, it feels like you’re dealing with four different versions of the same thing. But once you see where each code appears in the process, the confusion starts to fade. You’re not choosing between them — you’re moving through them. UPC or EAN gets your product into the system. ASIN is what that product becomes inside Amazon. And FNSKU is what allows your inventory to exist, move, and be fulfilled correctly inside FBA.
If something doesn’t work — a listing won’t go live, inventory doesn’t show up, or a shipment gets delayed — it’s often not because something is “broken”, but because one of these codes was used at the wrong step. Once you know what Amazon expects at each stage, the setup becomes much more predictable.
If you’re preparing to launch on Amazon EU and want to avoid these kinds of issues from the start, it helps to have someone who already works with this process daily. At FLEX Logistics, we support e-commerce brands with FBA preparation and fulfillment, making sure your products are correctly labeled, compliant, and ready to move through Amazon without delays. If you want to talk through your setup, you can book a call with our team and get clarity before your first shipment goes out.
