In IzPack, there can be used exceptions of type IzPackException in custom installer listeners which can be set to warning level. Currently, if there is registered a set of installer listeners, and during execution of these listeners there is thrown an IzPackException of type warning, a message box is opened with the question whether to continue the installation, but before that the execution of all subsequent installer listeners is implicitly skipped.
As far as I can see this currently doesn't affect built-in listener types, but it can be very useful for own listener implementations to inform the user but execute all other actions also of different listeners after the occurrence of a non-critical event.
The warning messagebox should appear like before but if the user decides to continue just skip the current execution (afterPack()... etc) and continue with the next one. If the user decides to abort the installation show a second messagebox with the information that the installation has been cancelled (in all interactive installer modes - GUI and console mode).