Two types of discount codes can be used in Discount Ninja:
Fixed: Link your promotion to a Shopify Discount code you have already created. To learn more about setting up discount codes in Shopify click here.
Generated: Creates a unique discount code for every customer that clicks the "Apply discount" button. By default the discount code is valid for 15 minutes. But you change that to any value (from 1 minute to multiple days) or to a fixed date. Find out more about how to set the expiration date here.