Credit Discounts give your client a flat dollar amount discount or free shipping on an order. This is the best type of discount to use if your client pre-paid for an order, or if you give clients a print credit with their session fee.
Credit Discounts are always Promo Code based, and require that the client enter a Promo Code in the cart or at checkout in order to receive the discount. If they don't use the entire amount of the discount when they check out, they forfeit the balance of the credit.