SKU

A stock keeping unit (SKU) is an organization’s identifier for an inventory item or variant it manages separately. Different sizes, colors, or pack configurations can have different SKUs when the business treats them as distinct stock. The code is not necessarily globally unique.

A red medium shirt and a red large shirt may have different SKU codes. Their quantities should not be combined when promising a specific size to a customer. Location, lot, and serial number may be additional dimensions rather than part of the SKU itself.

Keep stable mappings when codes change, and distinguish internal SKU codes from standardized trade-item identifiers and the barcode symbols that encode data. A well-formed code does not guarantee correct counts. For a metric, state whether the unit is distinct SKUs, item-location pairs, or physical units.

Related reference: Shopify inventory reporting documentation. Examples and calculations here are illustrative.