With the release of SKAdNetwork 4 (SKAN 4), Apple introduced a new system for conversion values that allow advertisers to track and report more granular conversion data, while still respecting user privacy. In SKAN 4, a fine conversion value refers to a conversion value is a 6-bit integer ranging between 0 and 63 (i.e. 64 values).
It’s only available for the first of three potential postbacks for an app install, and only if the marketing campaign that triggered the app install and postback meets Apple’s Crowd Anonymity standards. Postbacks two and three are always coarse conversion values: one of three different values.