SKUs are internal — each retailer designs their own SKU scheme. They differ from manufacturer codes (MPN) and global trade item numbers (GTIN/UPC/EAN), which are assigned externally and shared across all retailers.
A useful SKU scheme encodes information at a glance: brand, category, and variant in a predictable structure. For example, `WB-TSH-RED-L` immediately tells a warehouse picker the brand (WB), product type (T-shirt), color (red), and size (large). Pure-random SKUs (`a7f9e2b3`) are correct but unreadable.
SKU rules: never reuse a discontinued SKU on a new product, never change a SKU after launch (it breaks inventory history and reorder workflows), and always make SKUs unique across variants — never share a SKU between two variants of the same product.