Trumps lawyers are telling us they have no defense in the secret document investigation.

[FBaggins]

31. That's why is absolutely matters whether some of the documents are classified

He can't make just any document into a "personal record" - but there have been cases that say that the determination of personal/presidential is made entirely by the president. While it's almost certainly not the case that he can just say "those are all mine because I say so" - it's almost as certain that the government can't win in court with an argument that says "you can't take anything with you when you leave office and we get to decide what counts as a presidential record".

That would give the president a blanket claim to every document that he could think of and the whole damn archive if he wanted to as personal record.

It could in many cases, but not (I think) if they're classified. Which is why that matters. The courts (at least probably this SCOTUS) will come down somewhere in between the two extremes imagined above. But it would be much harder for them to rule that even classified documents are subject to the "Clinton sock drawer" test.

This isn't really a new area of law. Legal scholars have been speculating re: the borders here since it was first passed in the wake of Nixon. Like so many of TFG's actions, we get to watch what happens on the fringes of constitutional law in ways that no academic imagined would ever get out of their imagined world.