Cliff notes of this Erie Doctrine flow chart: Federal courts hearing a state law case because the parties are from different states have to use substantive state law but can use federal procedural law.
And how do you tell if a law is procedural? Well, you follow this handy-dandy flow chart, which is mostly useful, except for one part.
To quote the justices in Hanna v. Plumer, to tell if a law is procedural, you must ask the question, “Does the law *really* regulate procedure?”
I mean COME ON. What kind of guidance is that? I would love to hear those oral arguments: “Your honor, this statute is procedural because it really regulates procedure. Really really really really regulates procedure. Promise.”