Thank you for the clarification. Regarding your scenario, 'In the case of using the application (built with Unreal Engine) for promotional purposes for selling an unrelated product, would the revenue of said product be considered within the agreement with Epic', the answer would be no. Royalties would not be owed in this case. A contractor, work-for-hire arrangement to create a custom promotional product; work-for-hire fees have no royalty. If the application was sold to the general public, then there would be a royalty