How it works
A tracking link to Google Play carries a referrer parameter with details such as the click ID and the campaign. Google Play keeps that value when the user installs. On first launch the app's SDK calls the Play Install Referrer API and receives the referrer string together with two timestamps: when the click happened and when the installation began. The SDK sends this to the measurement platform, which matches it to the recorded click.
Why it matters for an app team
The referrer is a deterministic signal. It comes from the store itself, does not depend on the advertising ID and is not affected when a user resets or deletes that ID. The timestamps also expose fraud: a click recorded after the installation had already begun cannot have caused it.
In practice
Example: a user taps an ad at 10:00:00, Google Play starts the installation at 10:00:40 and the app is opened at 10:03. The referrer returns the click ID and both earlier timestamps, so the install is matched to that exact click.
Common confusion
The install referrer exists on Android only. iOS has no equivalent, which is one reason iOS measurement relies on SKAdNetwork. It covers installs that go through Google Play, so apps distributed through other stores or as direct downloads need a different signal. The API also replaced an older broadcast-based mechanism that was easier to abuse.