The jar Element
The IzPack jar xml tag allows you to specify external jar files containing a java archive that will be made available to all the panels in your IzPack installation. This is a top level tag that can be placed after resources and before panels. Use a separate jar tag for each jar file you want to include. The jar tag is useful if you write several of your own custom panels and multiple panels need access to a given java class.
The alternative to specifying the jar files with the jar tag is to unzip all of the associated class files and include them alongside the class files of each of your custom panels that require them. While this works, it causes code to be duplicated among each of the panels that require a given class file and prevents a given class instance to be shared between panels. For this reason, it is better and easier to include java class files that are to be shared between panels with the jar tag.