Overview

Every item listed on Amazon must be tagged with a unique stock‑keeping unit (SKU) that the seller creates. The SKU acts as the internal address for each product, linking inventory files, fulfillment workflows, and sales reports. Understanding how SKUs function is critical because a single mistake can overwrite data, generate duplicate listings, or cause costly fulfillment errors.

Key Points

• Merchant‑generated code — Unlike Amazon‑assigned ASINs or manufacturer UPCs, a SKU is a custom identifier that each seller defines for every product in their catalog.
• One‑to‑one requirement — No two distinct items may share the same SKU; each size, color, bundle, or variation needs its own separate code.
• Permanence after creation — Once a SKU is saved, it cannot be edited or reassigned; the only way to retire it is to delete the entire product record.
• Bridge to Amazon’s catalog — During inventory file uploads, Amazon matches the seller‑provided SKU to the appropriate product detail page, determining whether to update an existing listing or create a new one.
• Overwrite behavior — Submitting a file that contains a SKU already present in the seller’s catalog replaces the previous data for that SKU without additional warnings.

How SKUs Work on Amazon

1. File ingestion — When a seller uploads a flat file (e.g., .txt or .xlsx), Amazon scans each row for the SKU column.

   • Example: A file row with SKU “GADGET‑RED‑M” tells the system to look for that exact identifier in the seller’s inventory.

2. Match or create decision — The platform checks whether the SKU exists in the seller’s account.

   • If found: Amazon updates the existing product record with the new price, quantity, or description supplied in the file.
   • If not found: Amazon generates a fresh product entry and links it to the supplied SKU, establishing a new address in the catalog.