The Discount Ninja cart promotion code field accepts:
Promotion codes (Discount Ninja specific)
Shopify discount codes (percentage, fixed amount and free shipping are supported)
The cart promotion code field can be configured to allow the app to combine offers added via the promotion code field with those that are triggered automatically or via a link.
This allows you to ensure customers can either enjoy an automatic promotion or trigger one via the promotion code field, but not both.
This is configured in Settings > Building blocks > Cart promotion code field in the Behavior tab.
You can control if combining is possible for promotion codes and Shopify discount codes separately:
In both cases, you can configure the cart promotion code field to never or always allow such combinations. You can also choose to opt-in or opt-out.
Using opt-in or opt-out you can control which offers allow such combinations.
To configure this:
Open the offer
Find the Stacking tab
Tick the relevant checkboxes to opt in or out