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That is a recommendation, not a law. That’s what agencies are to do, the President can and does declassify at will, and a Federal Judge on a Bill Clinton case said that “sole, at will” power carries on after a President is out of office for Docs during his administration.
There are numerous pictures of Biden holding Clasified Docs outside a SCIF and the guy next to him is holding a cell phone.
Media has reported in the VP days that Joe Biden did Media Interviews with Classified Docs, open and strewn about his desk. Biden had no power to remove them from a SCIF.
https://www.ecfr.gov/current/title-32/subtitle-B/chapter-XX
And the various parts and sub-parts there, appear to be Federal Regulations - the law.
There are procedures left to the NARA, that are not yet clear, but in general, since Obama issuing some Executive Orders on the matter, specifically the marking of what is Declassified, may apply in Trump’s case.
Yet, no reason for the government to get excited about that. It is an administrative matter that could be resolved between the NARA and former President Trump, without all the Dept. of Justice theatrics.