Anything you own with a quantifiable monetary value. Yes, even if that value is not fixed.
> If I own a company worth X billion, does that mean I have to sell my shares to withdraw enough cash to pay my taxes, thereby tanking the value of my own company?
Yes.
> If I pay a lot in a downturn but my company does better in a boom, will I get refunded the extra tax I paid?
No. How does that even make sense?
The answers to these questions are easy, people you just don't like them, because they mean that those who have a lot and feel like they "got it fair and square" suddenly have to (gasp) contribute to society rather than hoarding all of that money to themselves. Even though you start to hit massive diminishing returns in terms of effect on your actual lifestyle once you're getting up into the million range, and see barely any effect at all of additional wealth or income once you're past, say, 100 million. It becomes just a dollar-denominated high score, so that you can feel smug about How Great A Businessman You Are, while millions of people starve on the streets unnecessarily.