*Trump has the power to declassify
*The procedure isn't clear, but the President determines the procedure.

Trump had the power to declassify. He obviously doesn't now - which is why it matters whether he did, in fact, exercise that power in the past.

The procedure is fairly clear - there's an Executive Order laying it out - but Trump could have changed that if he wanted to. There's no indication that he did, though. Which is why a court would probably require something in the way of evidence if he wanted to assert that he declassified something without following that procedure. The most obvious would be if he disseminated the information - if he discussed something that was previously classified on national television, for example, then that information might no longer be classified (though I don't think the rules on that are crystal clear, as people have gotten in trouble for circulating information that was already leaked).

One way to think about it is to compare it to his power to fire people. Most senior officials serve at the pleasure of the President - he can fire any of them at any time. But to fire someone, he would have to, you know, fire them. Tell them, "you're fired" - or tell someone else that the official is fired. Could the President just think to himself in the middle of the night, "I'm firing the Secretary of Labor," but not tell anyone that he did so? Would the Secretary be fired at the moment he thought it? Or is some action of the President, other than silent thought in his head, actually necessary to fire him?

Albaby