Discount codes come in two forms: a shared code anyone can use, and a unique code issued to one recipient. The technical difference between them determines what each can be used for.

Shared codes leak by design

A code that works for anyone will be shared, posted and indexed within hours of being issued, regardless of how narrowly it was distributed.

That leakage converts a targeted offer into a general price cut, and the seller loses margin on buyers the promotion was never intended to reach.

Shared codes are therefore used where broad reach is the point, and their economics have to assume that everyone who wants one will find it.

Unique codes bind an offer to a person

Generating one code per recipient allows redemption to be restricted to a single use, and often to the account or email the code was sent to.

The codes are produced in bulk from a randomised sequence, stored against the intended recipient, and marked as consumed the moment they are applied.

Randomisation matters because a predictable pattern lets outsiders generate valid codes without receiving one, which reproduces the leakage the mechanism was meant to prevent.

Validation happens at several layers

A code is checked for existence, for expiry, for prior use, and then against the conditions attached to it, such as minimum spend or eligible products.

Each of these can fail independently, which is why a code can be genuine and unused and still be rejected at checkout for reasons the shopper cannot see.

The order of these checks also determines the error message shown, and many systems return a generic failure rather than reveal which condition was not met.

Codes are revocable in a way prices are not

Because validation happens at the moment of use, an issued code can be disabled centrally at any point before it is redeemed.

That allows a seller to stop a promotion that is performing badly, or to cancel codes obtained improperly, without any change to the underlying prices.

The same mechanism enables budget caps, where a campaign stops accepting codes once total redemptions reach a set level.

What the tracking is really for

Each redemption ties a discount to a specific basket, which allows the seller to see what was bought alongside the promoted item and whether the customer returned later.

Aggregated across a campaign, this produces a measurement of what the promotion actually generated, rather than an estimate drawn from overall sales.

The measurement often outlasts the promotion in value, because it informs how the next offer is targeted and how large it needs to be.