Terminology
We refer to the following field with the term Shopify discount code field:
We refer to the following field with the term Discount Ninja promotion code field:
How can these fields be used?
Shopify discount code field
Only accepts Shopify discount codes, does not accept Discount Ninja promotion codes
You can use only 1 code
Can be configured to stack Shopify discount codes on top of Discount Ninja promotions
Available on the checkout form (step after cart)
Managed by Shopify
Discount Ninja promotion code field
Can be configured to accept both Discount Ninja promotion codes (set up in the Trigger tab of your promotion, see more here) and standard Shopify discount codes.
Can be configured to accept multiple codes
Available on the cart form (step before checkout)
Managed by Discount Ninja
Needs to be added to your theme and enabled in the settings. Read more here.
When will Discount Ninja show the Shopify discount code field?
When no Discount Ninja promotions apply: Shopify takes over and shows a discount code field if one or more discount codes have been configured in Shopify. (this looks like the case from your screenshot)
When one Discount Ninja promotion applies that is in fact a wrapper around a Shopify discount code (promotion type "Apply discount", not free shipping), the same happens, but Discount Ninja passes the discount code to the checkout, which is handled by Shopify.
In all other cases (when one or more Discount Ninja promotions apply) you have the choice whether you want to show it or not. This is managed using the stacking options in the promotions and the behavior options of the promotion code field. See this article for more information.