The CheckedHelloPanel displays acts like the HelloPanel, displaying a welcome message including project name, the version, the URL as well as the authors in a built-in standard format(Described where?).
For Windows, it also scans the Windows registry for an entry that determines whether the product is already installed. It uses the appName specified in the installation file and searches the registry for XXXX.appName For this purpose, the elements <appname> and <appversion> in the <info> element of the installation definition file, in the for
$appName $appVersion (divided by a space character).
If the product was previously installed installed, a dialog will be shown which asks whether to install a second version of the product or not.
If you use this panel do not forget to add the COIOSHelper registry feature into your installation, see Windows Registry Access for more details.
See also RegistryInstallerListener / RegistryUninstallerListener for getting an idea how this information in the registry can be created during an IzPack installation and deleted during uninstalling.