Discount depth is how far below the list price a promotion sells, as a share of that price. Depth matters more than frequency: because contribution is a thin slice of the price, a ten percent discount typically removes a quarter of what the order was keeping, and twenty percent removes about half of it.
The leverage is what surprises people. When roughly a third of the net price survives goods, parcel and fees, every point of discount comes out of that third rather than out of the whole, so the required volume to stand still rises far faster than the depth suggests.
Depth also decides whether a promotion is a marketing decision or a liquidation. Below the price that covers goods, parcel and fee, the store is paying customers to take stock away, which is occasionally the right call for dead stock and never a strategy.
Measuring it honestly means measuring the effective depth: the average across everything sold in the promotion, including full-price orders that used the code anyway. A sitewide code gives its depth to customers who were already in the checkout, and that share buys no additional volume at all. Whether discounted orders really are larger is tested on real data in discounted orders are bigger orders.
