This panel allows choosing between two installation types:
normal installation (default, if modify.izpack.install not set or false)
modifying a previous installation
For this purpose it presents two radiobuttons to the user:
It uses the reserved internal IzPack variable modify.izpack.install as input and output for controlling the behavior of other panels, like the TargetPanel.
Further, it may check the $INSTALL_PATH/.installationinformation file, which must have been written by a previous installation to be able to modify it.
If "Modify installation" is chosen the installers loads packs already installed in a previous installation and removes them from the packs to be actually installed. Therefore, on subsequentially opening a PacksPanel, there is just the set of packs presented to the user, which actually haven't been installed to this time.
Further this has influence on PathInputPanel and TargetPanel, requiring them to not only check the existing target installation path, but also the .installationinformation file. The installation is not allowed to go on if this condition is not fulfilled.
The saved installation information is very fragile and every change in packs or variables can break the update functionality of an installer for a dedicated information. Just a basic merging is done for packs and overridden variables can lead to unexpected behavior. It is not recommended to use this feature for more complex installers. The .installinformation file depends on Java serialization which might change between JRE and IzPack versions.
Can be initially set in the <variable> section to change the default settings on opening this panel first.