Whether Shopify allows selling a variant with zero on-hand stock.
Each variant has an inventory policy of either `deny` (default) or `continue`. `Continue` allows customers to place orders while inventory is at or below zero — useful for preorders, backorders and dropshipped items with delayed feeds, dangerous for finished goods where oversell means refunds and reputation damage. Never toggle `continue` in bulk without a matching pre-order badge and fulfilment plan.
