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Hello everyone. I'm having to explain to my friend what the phrase "what is that all about?" means (or, more precisely, when and how it should be used), and it's proving to be much more difficult than I originally thought. Hence, I need help from someone to fully understand what the phrase actually means. My main question is, is the word "all" used as an adjective, or an adverb? I think knowing that would help me a lot.
I think this is rather an odd question, but I would be greatful if someone could help me out. Anecdotally, I know when and how to use the phrase, but being able to explain that to someone is happening to be an entirely different matter.

Thanks a lot in advance.

Vocab
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Try turning it into a statement:

That is all about the war in Iraq.
That is wholly about the war in Iraq.

All is an adverb.
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> "what is that all about?"
What is the subject there (in that matter/discussion)?
What is the point?