A SKAN conversion value is a small number your app sends Apple to describe what a new user did right after installing.
SKAN postbacks carry no names, so the one lever you get is the conversion value. Your app updates this number as the user hits early milestones, and Apple folds it into the anonymous postback. In SKAN 4 you get a fine-grained value in the first window and a coarse label of low, medium, or high in the later windows, so you can still tell a strong cohort from a weak one without any personal data.
Say you decide value 0 means just opened the app, a middle value means finished signup, and a high value means made a purchase on day one. A campaign whose postbacks skew high is bringing better users than one that skews toward 0, even though you cannot see a single individual. Open your SKAN conversion schema and check what each value maps to; if it is still the SDK default, you are throwing away your only signal.
You have to design the schema before the campaign runs, not after. The mapping is locked for the users it measures, so a lazy or wrong schema quietly costs you signal on spend you cannot rerun.
The conversion value is the one sentence you get to whisper to Apple, so make it say something.