How it works
When an ad leads to an install, Apple sends a signed postback to the ad network (and optionally the advertiser) with campaign information — but without device IDs. Postbacks are delayed and aggregated, and some data is withheld for small campaigns to protect privacy.
Apps can report a limited conversion value to tell advertisers what the user did after installing (e.g. completed onboarding, purchased). SKAdNetwork 4 added multiple postbacks over time and coarse values for smaller campaigns.
AdAttributionKit
From iOS 17.4, Apple introduced AdAttributionKit, which builds on SKAdNetwork and also supports alternative app marketplaces. It is interoperable with SKAdNetwork.
Why marketers care
After App Tracking Transparency, SKAdNetwork-style data is often the only attribution available for iOS users who don't allow tracking. Planning conversion values well decides how useful the data is for user acquisition — usually configured through a mobile measurement partner.
Example
A subscription app sets conversion values to encode "started trial" and "converted to paid" in the first days, so ad networks can optimize for subscribers rather than installs.