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Setting displayHidden="true" means "show read-only in case it is otherwise hidden instead of hiding it according to its condition". This is a special use case, to show panels or fields not enabled for user input, but which are otherwise valid for the installation process. Using this approach you can simply reuse panels or fields and switch single fields to read-only mode instead of hiding it completely. The advantage is being able to automatically switch to read-only mode depending on the effective panel or field condition without having a dedicated condition for the panel or field to switch to read-only mode.
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