Secret deals, six-figure payoffs, salacious stories -- the testimony of former National Enquirer publisher David Pecker in Donald Trump’s hush money trial this week has offered a rare window into the tabloid practice of checkbook journalism, where a publication pays its sources.

A scheme to “catch and kill” unflattering stories about Trump lies at the heart of the prosecutors’ case, alleging Pecker arranged to pay sources for the rights to such stories to suppress them.

Trump has been charged with 34 counts of fraud tied to his alleged role in payments made to porn star Stormy Daniels as part of a “catch and kill” agreement. Trump has pleaded not guilty and denied any wrongdoing. He has also denied having sex with Daniels.

Experts who spoke to ABC News described a narrow set of tabloid publications in the U.S. that offer direct payment for source cooperation, explaining why the practice is widely condemned but still continues.

Here’s what to know about checkbook journalism and the role it plays in the hush money trial:

How did the National Enquirer pay for stories and why does it relate to the case against Trump?

The hush money trial has unearthed fresh details about the checkbook journalism overseen by Pecker, who served as CEO of National Enquirer parent company American Media Inc. from 1999 to 2020.

Editors at the magazine could spend up to $10,000 to get ahold of a story, but any payment exceeding that amount required sign-off from Pecker, he testified.

Pecker said that he retained "the final say on the celebrity side of the magazines."

“We used checkbook journalism and we paid for stories,” Pecker added.

Pecker established a "secret arrangement" with Trump and his then-attorney Michael Cohen during a "20-25 minute meeting" at Trump Tower in August of 2015, Pecker testified.

Pecker testified that he suspected that multiple women would come forward to shop stories about Trump during Trump's run for president. If those stories emerged, Pecker said he would notify Cohen, per their agreement.

Cohen also agreed to provide opposition research to Pecker for negative stories on Trump’s opponents, Pecker said.

According to Pecker, most elements of their agreement -- including running positive stories about Trump and negative stories about his opponents -- were "mutually beneficial" to Trump and Pecker.

"It would help his campaign, but it would also help me," Pecker said.

How commonplace is the practice of paying sources?

In his opening statement at Trump’s hush money trial, defense attorney Todd Blanche said the National Enquirer’s practice of paying sources was in keeping with standard journalistic practice.

In U.S. media, the practice of paying sources for stories is largely confined to tabloids like the National Enquirer, experts told ABC News, rejecting the notion that checkbook journalism is carried out across the industry.

“It’s definitely not within the bounds of journalistic practice,” Jane Kirtley, a professor of media ethics and law at the University of Minnesota, told ABC News.

The alleged “catch-and-kill” scheme is especially egregious, Kirtley added. “Journalists exist to report the news, not suppress it,” she said.

The approach of paying sources for news is widely condemned by media ethics organizations and major news outlets.

The Society of Professional Journalists, a nationwide organization representing journalists, condemned checkbook journalism in a statement on Tuesday in response to testimony in the hush money trial.

“It is clearly unethical,” SPJ National President Ashanti Blaize-Hopkins said in a statement.

Is it illegal to ‘catch and kill’ news stories?

While frowned upon by many in the field, checkbook journalism is perfectly legal -- and that includes the “catch-and-kill” scheme involved in this case, experts said.

Instead, Trump stands accused of falsifying business records as part of the arrangement to reimburse Cohen for the hush money payments to Daniels. According to prosecutors, Trump fraudulently recorded $130,000 in expenses as the cost of legal services for Cohen.

"This is the business capital of the world," Manhattan District Attorney Alvin Bragg said at a press conference announcing the charges last year. "The bedrock, in fact the basis, for business integrity and a well-functioning business marketplace is true and accurate record keeping."

Prosecutors called up Pecker for testimony in part as a means of bolstering allegations that Trump played a role in the scheme involving the payments.

Checkbook journalism is protected by the First Amendment, John Watson, a professor of journalism at American University who focuses on ethics, told ABC News, noting that the press enjoys wide latitude around how and what to publish.

"The First Amendment protects any sort of expression, outside of obscenity," Watson said, while acknowledging other exemptions for false and defamatory speech.

The legality of checkbook journalism, however, makes up a separate consideration from the question of whether it's ethical, Chad Painter, a professor of media ethics at the University of Dayton, told ABC News.

"Ethics goes further than the law," Painter said. "There are things that are perfectly legal but we're still not going to do them -- in terms of media or us as people."

ABC News' Aaron Katersky, Peter Charalambous, Olivia Rubin, Lucien Bruggeman and Julia Reinstein contributed to this report.