Lawyers for former President Donald J. Trump and prosecutors who have accused him of illegally retaining national-security documents after leaving office are expected to meet on Tuesday to discuss how to handle classified materials at the heart of the case.

The hearing in front of Judge Aileen M. Cannon at a federal courthouse in Fort Pierce, Fla., could touch on other issues, including the scheduling of the trial. But it may be an early step in what is expected to be a lengthy closed-door fight under a law that regulates the use of secret evidence at public trials: the Classified Information Procedures Act.

Here is a closer look at the tricky legal issues raised by the role of classified evidence in the case.

What are Espionage Act charges?

Mr. Trump faces 31 counts of violating the Espionage Act, which makes it a crime to mishandle national security secrets. To prove that Mr. Trump violated the charged provision of the act, prosecutors must show that he possessed, without authorization, closely held information “relating to the national defense” that could harm the United States or aid a foreign adversary, and that he failed to return it to the government.

Each count is based on a different sensitive document the F.B.I. found in its court-authorized search of Mr. Trump’s club and estate, Mar-a-Lago. As described in the indictment, they include 21 marked “top secret,” nine marked “secret” and one without a classification stamp that contained restricted information about “military contingency planning.”

What is ‘graymail’?

It is a threat by a defendant to reveal classified information during a trial in the hope of forcing the government to drop a criminal charge. While the government could choose to declassify such information so that it could be freely discussed in open court, security officials may see that as too risky. But the Constitution gives defendants a right to a public trial and the public a right to see trials.

Joshua L. Dratel, a defense lawyer with a security clearance who has handled terrorism cases involving classified evidence, said defense and intelligence officials often did not want to give up information for prosecutors to use in pursuing a case.

“It’s routine, if not invariable, that you’ll get a plea offer in a case in which the government says, ‘If we have to provide classified discovery to you, this offer is no longer on the table,’ and those can be very attractive plea offers,” Mr. Dratel said. “That’s because there is a tremendous tension between intelligence agencies and prosecutors that defense lawyers can exploit.”

Mr. Trump, however, is not expected to accept any plea deal.

How could classified evidence be an issue in Mr. Trump’s documents case?

The jury presumably needs to see at least parts of each of the 31 files singled out in the indictment to evaluate whether they meet the standards of the Espionage Act. But the issue is not likely to be limited to just those records. Defense lawyers may also ask the government to turn over related classified evidence in the discovery phase, and then seek to use some of it at the trial.

In a court filing in July, Mr. Trump’s lawyers suggested that they would push hard to make public as much as possible of the sensitive evidence in the case.

“In general, the defendants believe there should simply be no “secret” evidence, nor any facts concealed from public view relative to the prosecution of a leading presidential candidate by his political opponent,” the lawyers wrote. “Our democracy demands no less than full transparency.”

How will this raise logistical hurdles?

Even to be in a position to fight about such issues, defense lawyers in the case will face some challenges.

For one, classified documents must be kept in a so-called SCIF, or Sensitive Compartmented Information Facility, which complies with certain security rules. The government maintains one for cleared defense lawyers to use at the federal courthouse in Miami — not Fort Pierce — and said in a court filing last week that it would bring the materials there.

For another, Mr. Trump will need representation by lawyers with security clearances. The government’s filing this month said that only two defense lawyers had applied for an interim clearance so far. It indicated that such a clearance would permit them to see “the large majority,” but not all, the documents with classification markings recovered from Mar-a-Lago.

Once defense lawyers have final security clearances, the filing said, the government will bring the remaining such files to the SCIF in Miami, along with statements of witnesses who discussed classified information and “classified records pertaining to the declassification of various materials during the Trump administration.”

What is CIPA?

Congress enacted the Classified Information Procedures Act, or CIPA, in 1980 in an effort to reduce the chances that graymail would derail prosecutions of people in cases involving national security secrets. (Coincidentally, the bill was introduced in 1979 by Senator Joseph R. Biden Jr., who was the chairman of the Judiciary Committee.)

CIPA established ways for prosecutors, defense lawyers and judges to frame classified information so that it could be used in public without compromising protected information like sources and methods. Before a trial even begins, litigation involving the statute typically plays out behind closed doors.

In the documents case, Judge Cannon must agree beforehand that any proposed use of CIPA would not infringe on Mr. Trump’s right to a fair trial.

“It is a very complicated lengthy process,” said Barry Pollack, a defense lawyer who also has a security clearance. “Often, there are hearings not open to the public where the attorneys and the judge will literally go through documents line by line deciding which sentences and which individual words can be used in open court and which ones cannot.”

How does CIPA work?

It allows a court to block, censor or create substitutions for classified evidence under certain circumstances.

Prosecutors can seek to use the law to limit evidence they turn over to the defense in the discovery phase. And defense lawyers must tell the judge and prosecutors before the trial what classified evidence they intend to introduce, making the case that it would be material and explaining how they plan to use it.

Judge Cannon could block such evidence, allow the government to redact portions of it or permit a substitution that provides the gist — so long as she decided that doing so would not impede Mr. Trump’s right to a fair trial.

For example, during a 2013 case when Mr. Dratel represented a San Diego man accused of sending money to a Somali terrorist group, prosecutors gave him a summary of the government’s intelligence about the group’s fund-raising. Both sides then agreed on a statement to show the jury at trial reciting certain facts without revealing how they were learned.

As in other such cases, defense lawyers could object to proposed redactions or substitutions, arguing that the full details are necessary. The prosecution can appeal Judge Cannon’s decisions before the trial, but the defense would have to wait until after any conviction.

What is the silent witness rule?

It is a judicially created process that is similar to CIPA substitutions with one key difference: The jury sees the classified evidence, while members of the public in the gallery get less information. By contrast, under CIPA, both the jury and the public see the same thing.

Under the silent witness rule, for example, a witness and the jurors could be handed a document that is still classified, but it would not be made available to the public. During testimony, the witness might refer to “the matter in the document’s third paragraph.” By looking at the document and following along, the jurors would know what the witness was specifically discussing but onlookers would not.

How legally established are these procedures?

Some appeals court rulings have validated CIPA in various contexts, such as rejecting arguments that it deprived the defense of information to which it should be entitled or that it violated a defendant’s right to confront his accuser. The Supreme Court has never addressed the law, which could give Mr. Trump a basis to appeal any conviction if the trial used redacted or substituted evidence under that law.

There is even less precedent about the silent witness rule. The Court of Appeals for the Fourth Circuit, in Richmond, Va., last year approved its limited use in a case accusing a former intelligence official of violating the Espionage Act by giving restricted information to China. But Judge Cannon is not bound by that precedent because she is overseen by the appeals court in Atlanta.

Alan Feuer contributed reporting.