I think it was a nuclear weapons related program that had some major development milestone during his presidency, and so- as is his way- he felt he was responsible for it... that he OWNED it...

I keep coming back to this, but remember he bragged about a new nuclear weapons development to Woodward:

“I have built a nuclear — a weapons system that nobody’s ever had in this country before. We have stuff that you haven’t even seen or heard about,” Trump told Woodward, according to the Post.

“We have stuff that Putin and Xi have never heard about before,” Trump added, referring to Russian President Vladimir Putin and Chinese President Xi Jinping. “There’s nobody — what we have is incredible.”

God help him if he divulged it to anyone, on purpose or by accident... god help us all...