Reattribution is when a user re-install a mobile app thanks to a re-engagement campaign. A re-install event from a re-engagement campaign is not counted as a new install and a new postback is not created for this install. 

A reattributed user is a returned user who once uninstalled an app and doesn’t use it for a long time. Churned users who have not had any bad experiences are qualified target audiences for mobile app developers and mobile advertisers because they showed interest in the app at least for a specific time period. Re-engagement campaigns help to convert those users again and reattribution models assign the re-installs to the original attribution source.

Reattribution window indicates the time period between re-installing of an app and re-assigned the install to the original attribution source (the source of the first install)