Discount Ninja in general does not use Shopify discount codes to apply discounts to the checkout

However, the free shipping promotion type is an exception. In this promotion template we allow you to select an existing discount code from your Shopify discount module or instruct the app to create a new Shopify discount code for you. This happens on step 2 of the wizard:

Regardless of the option you choose, the app will then start to add this new/existing discount code to the checkout when the promotion is triggered.

When you make changes to the promotion in Discount Ninja we will automatically update the discount code in Shopify. To avoid that, edit your offer after setting it up and toggle the following setting OFF in the Discount tab:

Changes made in the discount code in Shopify will be overridden by the settings in the promotion to ensure the promotion and discount code remain in sync. When you edit your discount code in Shopify, scroll down to the bottom to see the audit trail and confirm the app has reverted your changes:

To avoid that, edit your offer after setting it up and toggle the following setting OFF in the Discount tab:

When you rename the discount code in Shopify, the app will revert the change when the promotion is saved or used. 

You can rename the discount code in Discount Ninja. Edit the offer and find the Discount tab. Click the rename button:

When you disable and delete a discount code in Shopify, the app will show an error message:

On the "Discount" tab, select a new discount code.