It really isn't ill-defined at all. Both the constitution and the law allowing the president to impose tariffs for national security reasons is clear. There are just some partisan hacks on the Supreme Court.

This specific law does not allow imposing tariffs, which is the whole point of the ruling. Roberts’s opinion says that a tariff is essentially a tax, which is not what Congress clearly delegated.

Wrong law. Trump chose not to use the "impose tariffs for national security reasons" law in this case.