Shopify has a relatively new discount code type “Buy X Get Y“.
You can now export and import that discount code using the Matrixify app.
The best way for you to understand which columns need which values are if you enter one such Discount code manually in the Shopify Admin, and then do the export using the Matrixify app.
Then you will see what data goes into which columns.
See all the documentation about all the Discounts sheet columns.