A one-digit correction to President Obama’s directive on hostage policy Wednesday had the effect of disclosing the existence of a previously unknown — and still-secret — Obama order on national security.

The hostage policy was originally released Wednesday as a presidential policy directive numbered PPD-29. When the White House corrected that number to PPD-30, it meant Obama had issued a secret directive as PPD-29 sometime in the past 17 months.

Obama signed PPD-28, an order on electronic eavesdropping in the wake of revelations by Edward Snowden, in January 2014.

So what is PPD-29? No one’s talking. A spokesman for the National Security Council declined to comment of the existence of classified PPDs Wednesday.

“The only reason we know about it is the sequential numbering of the directives, and realizing they skipped a few,” said Steven Aftergood of the Federation of American Scientists, which tracks the directives.

PPD-29 isn’t the first to be tacitly acknowledged only by a missing number. Of the 30 PPDs issued by Obama, 19 have not been released. And for 11 of those, the White House has not disclosed even the subject of the order.

“It’s not only the public that doesn’t have copies. It’s also Congress that doesn’t have copies,” Aftergood said. “It’s a domain of largely unchecked presidential authority. It doesn’t mean it’s bad, but it’s lacking in independent oversight.”

But they have the same legal force as an executive order, forming a body of largely secret law, said Harold Relyea, a political scientist who advised Congress on national security directives before retiring from the Congressional Research Service.

“The difference is that while executive orders are public by law — they must be published in theFederal Register to be effective —- PPDs are not,” he said. “It is a kind of secret law. People have to obey it. But it’s a directive that can allocate money, direct people or take a course of action.”

What Obama calls PPDs have gone by different names by different presidents back to the Truman Administration. President George W. Bush called them National Security Presidential Directives (NPSDs). President Clinton called them Presidential Decision Directives (PDDs). President Nixon called them National Security Decision Memoranda.

Whatever they’re called, Obama has been less prolific than his predecessors. George W. Bush issued 66 such orders, plus 25 more Homeland Security Presidential Directives. President Reagan issued at least 325.

Some, going back as far as the Lyndon Johnson administration, remain classified. They can involve subjects including the use of nuclear weapons, ballistic missile defenses, space policy, cybersecurity and even continuity plans for the federal government in the case of a large-scale disaster.

The secrecy makes it difficult to know entirely what changes Obama has made in the hostage policy. The directive issued Wednesday revoked a prior directive by President George W. Bush in 2002. But that directive, known as NSPD-12, remains secret.

“You would think that if there’s a new policy it would be a simple matter to explain what the old policy was,” Aftergood said.

And even though Obama released his directive, it incorporates a classified annex with additional instructions to executive branch agencies.

WASHINGTON