Vendor catalog: item identity, packaging hierarchy, and account context

• ASIN — Amazon's catalog identifier, shared with every seller and with Amazon Retail. The same ASIN can be supplied by a vendor and sold by third parties at once.
• UPC / EAN / GTIN — the GS1 barcode of the consumer unit. Cases carry their own GTIN-14 (ITF-14) barcode; never put a case code in the unit field.
• Vendor code — the short code that identifies your vendor account (one company can hold several: by country, category, or legal entity). Every item lives under a vendor code, and cases, POs, and support are scoped to it.
• External ID / vendor SKU — your own item number. Amazon stores it so POs and reports can be matched back to your systems.

Keep the four together in one row of your source of truth. A support case that cites the ASIN but the wrong vendor code goes nowhere.

Vendor items are set up as a hierarchy, and the fields are not interchangeable:

• Each (consumer unit) — what the customer receives. Its GTIN, dimensions, weight, and content describe one sellable unit.
• Case — how you ship to Amazon's fulfillment centers. Case quantity, case GTIN-14, case dimensions and weight.
• Pallet — layer and tie-high configuration for large volumes.

A capacity that is right for the unit is wrong for the case, and vice versa. Amazon uses case data to write purchase orders and to route inbound freight, so a unit dimension entered in the case field produces chargebacks and refused shipments, not a listing error.

Before using any workflow, confirm three things: which vendor code you are acting under, which marketplace (amazon.com, .co.uk, .de… each is a separate Vendor Central), and whether the ASIN is already in that catalog. Vendor Central's account switcher makes it easy to submit a perfectly correct item to the wrong account.

In Items → Add Products, search by UPC/EAN first. If the ASIN already exists — because a third-party seller created it, or because it is already in another of your vendor codes — you add it to your catalog rather than creating a duplicate. Amazon merges duplicate ASINs eventually, and the merge can take your content and your purchase-order history with it.

Only when the search returns nothing do you proceed to new item setup: choose the product type, complete the required attributes, provide the unit and case data, and submit for approval.

• Items → Catalog — every ASIN under the vendor code, with availability status.
• Items → Add Products — search, add existing ASINs, or set up new items one at a time.
• Items → Bulk product submissions (new item setup spreadsheets) — set up or update items at scale, and check submission status.
• Items → Edit Products — change detail page content for ASINs already in your catalog.
• Support → Contact Us and the Case Log — everything that cannot be done in a feed.
• Support → Resource Center — Amazon's own vendor guides, behind login.