Buy X, get Y free (different SKU)

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For this rule you need to define more in detail which products need to be purchased (Conditions) and which ones will be for free (under Actions).

If you want to offer "Buy two shirts, get one hat for free", you would define either the shirts category or the SKUs in your condition rules, and the hat category or SKUs under Actions. Set the Discount amount to 100 for Percent of Product Price Discount.