This generates an installer that can only be installed on top of an installation of a certain installation. An add-on installer doesn't have a separate uninstaller. This is useful to distribute patches and enhancements.
If the add-on installer type is selected, you have to enter an application ID for the base application in the text field below. With the [...] chooser button, you can select an install4j project file from your file system, and extract its application ID.