Plesk

Application Vault – Tracking App Installations

In general, customers install apps directly from the Application
Catalog, bypassing the local repository. The only exceptions are apps
that you have added to the Vault. Nevertheless, all app installations
are registered in the Vault. You can view the details on the
installations in the Tools & Settings > Application Vault > Installed
Apps
tab. A zero number of app installations in the list means that
the app package is stored in the Vault but is has not been installed by
any customers.