How it works
There are two kinds. A fine conversion value is a 6-bit number from 0 to 63. A coarse conversion value has three levels: low, medium and high. The advertiser decides what each value means, for example revenue ranges, a step in the funnel or a mix of both, and the app or its measurement SDK updates the value as the user acts. In SKAN 4 the first postback (days 0–2) can include a fine value when enough installs stand behind it to pass Apple's crowd anonymity threshold; otherwise it carries a coarse value or none. The second and third postbacks (days 3–7 and 8–35) carry coarse values only.
Why it matters for an app team
The mapping decides what ad networks can optimise for on iOS and what you can read from SKAN reports. A schema that spends its 64 values on events nobody acts on leaves you with install counts and little else.
In practice
Example mapping, invented for illustration: values 0–9 mean the user opened the app and reached a given onboarding step; values 10–63 mean a purchase, each standing for a revenue range, such as 10 for up to $1 and 11 for $1 to $2. For coarse values, low is an install, medium a registration and high a purchase.
Common confusion
A conversion value is neither revenue nor an event. It is a code that only makes sense together with your mapping, so changing the mapping in the middle of a campaign makes older and newer postbacks incomparable. A postback without a conversion value does not mean the user did nothing; it usually means the privacy threshold was not met.