To use this feature, you simply edit a file in your Control Panel to add or remove Certificate or Coupon numbers and their corresponding discount or dollar value.

You may designate the discount coupons as either percentages or flat amounts.
The discount will apply on the subtotal of their order but not on their sales tax and shipping. So they will still pay for any applicable sales tax and shipping.

Example:
If a customer enters in a valid Coupon secret number, they will receive a discount based upon whatever percentage you have pre-determined on their order.

Example
If a customer enters in a valid Gift Certificate number they will receive the amount of the Gift Certificate off their total price.