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IzPack installers use Java Logging for logging any information. If you want to customize it you should be familar with it, otherwise there is a good description of its principles in Oracle's Java Logging Overview.
Without any explicit specification on the command line or in the install.xml itself, an installer logs to the console at the given log level. For this purpose it associates a default Java ConsoleHandler combined with a built-in formatter not being that messy like the JRE's SimpleFormatter. The log level to limit the output to depends on whether you set IzPack to debug mode by adding the command line option -debug (or the -DDEBUG=true JVM option for IzPack versions < 5.1.0). If the installer runs in debug mode the Java Logging level for the root logger is set to FINE by default globally, by default it is INFO otherwise.
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