Uninstalls occur when the user uninstalls an app after receiving a campaign. Opt-outs occur when push is disabled for the app; this prevents the user from receiving the campaign.
Uninstalls are determined by sending what are known as "ghost push" messages to a device. A device is considered to have uninstalled when the push notification service indicates that the push token is no longer valid (technically, "NotRegistered" on Android, or listed by the Apple Feedback Service as no longer active on the user's device). This is the industry standard for determining a device uninstall.
Kahuna calculates this event as an uninstall event. The Reach page displays the number of uninstall events received, which is not the same as the number of uninstalled users, since the user can reinstall after uninstalling, or uninstall multiple times, or have several devices. It is also possible that the event is a false positive, in that the user will obtain a new push token after the uninstall. In this case, we do not modify the number of uninstall events, but do re-enable pushes for that user's device.