The blurb under the headline on the main page.

Customers earn store credit automatically when a signed-in customer's order is marked "Sold" — a percentage of the quoted price, added to their balance. They can use it toward future orders (you apply it per-order on the order card).

Codes customers can enter when placing an order. Entering one doesn't change any price automatically — like store credit and the signup discount, you see it on the order card and factor it into the quote you type in.