SKAdNetwork conversion value (integer in the range 0 to 63), the meaning of which is determined by the ad network or the app. The conversion value lets you determine events after conversions and evaluate the campaign results.
Unknown means that an ad network does not pass conversion value, or you build a report for data not related to SKAdNetwork, for example, for Android apps.
SKAdNetwork is Apple's framework that helps to measure the success of ad campaigns while maintaining user privacy. Learn more