How discounts work
One pricing engine decides every discount, the same way, on every channel.
User guide
One pricing engine decides every discount, the same way, on every channel.
Every discount in SellerTower is priced by one engine, on the server — the same rules at the back-office checkout, the POS and the storefront. What you see in a checkout preview is what will be charged, because both come from the same calculation; no screen does its own discount arithmetic.
Who can do this: authoring discounts is the Admin's (Store Admins can view them); a cashier applies them at the till through their order permissions. Discounts are a plan capability — the badge above says where they start.

An automatic discount the cart doesn't qualify for is simply skipped, silently — shoppers are not nagged about promotions that don't apply to them.
A discount someone asked for — a typed code, a selected discount — that fails gets an explanation: the checkout lists each rejected discount with the engine's own reason ("discount is expired", "minimum order value of X required", "you have already used this discount the maximum number of times"…). An ineligible requested discount on a new order is refused outright rather than quietly dropped, so an order is never placed on the belief a code worked when it didn't.
A discount's redemption is counted after the order is actually saved — a failed checkout never burns a single-use code. The count is given back only if the order is cancelled; a return keeps it, because the promotion did make the sale.