Those predefined tags are defined within an XML schema describing what parts needs to be present in the file for an interpreter to make sense out of it in a specific context.
XML itself has no predefined tags. It's when you start using it you define what tags you need and what tags are optional.
If you write your own application you might demand that the tag <ApplicationRegistryKey> is present. But that will not make it predefined in XML. Just in your schema.
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