Variant matrix
A variant matrix is the full set of sellable combinations of a product’s options, each of which is a distinct purchasable item carrying its own price, availability and identifier rather than inheriting the parent product’s.
A garment in six sizes and four colours is twenty four sellable items, not one item with two dropdowns. Whether that is published as twenty four records or computed in the browser is the difference between a product an agent can buy and one it can only look at.
The modern failure is a picker that fetches the matrix from an API on load. The page renders correctly and the product is variantless to anything that read the markup.
Why it matters to a store
This is the most expensive place to fail, because everything before it succeeded: you were found, read, compared and chosen, and the purchase ended at a dropdown.