The best explanation I heard was an interview with Mike Pompeo. He said if you leave a classified document on your desk you get disciplined. If you take one home you get fired. And if you take one home and someone else sees it either intentionally or unintentionally you get prosecuted. Obviously he was painting with a broad brush but on that basis Biden should be in a world of hurt.
Did you see the emails Hunter was sending that contained very strategic sounding information about Ukraine and the UK? I am betting we could probably match it up to classified documents in the garage or wherever the big guy stored the stolen docs. If so, that fits the third case although given how cavalier he was with the storage of all these documents who knows how many people saw them. And where were the ones at UPenn in the year or more between him leaving office and them opening up the ChiCom money laundering office at Penn?