Promo Codes can be set up as a % off or a $ amount off.
For a % Percentage off
When setting up the Promo Code enter the % Percentage off in the Enter the Discount Percentage field.
For a $ Dollar Amount off
When setting up the Promo Code leave the Enter the Discount Percentage field blank and enter a $ dollar amount off in the "Enter the Discount Amount" field.
With a % Discount you can cap the $amount of the discount to be used by entering the Discount Percentage and enter a maximum amount in the Discount Amount.
For instructions to configure Promo Codes select the following link Configuring Promo Codes