SKU
A SKU is the identifier a retailer assigns to a sellable item for its own stock control. It is unique within that business and meaningless outside it.
This is the distinction that causes trouble in feeds. A SKU tells your warehouse which item to pick; it tells nothing outside your business which product this is, because another retailer’s SKU for the same item is different.
It still belongs in a feed as the stable per item key, and it does not substitute for a GTIN or an MPN when the question is whether two listings are the same product.
Why it matters to a store
A feed carrying only SKUs is internally consistent and externally unmatched. Every listing is a distinct unknown, so nothing can be compared against a competitor selling the same thing.