The U.S. Justice Department is opposing a request by former President Donald Trump to appoint a "special master" to determine whether documents seized by federal agents from his Mar-a-Lago home on Aug. 8 contain materials that are covered by attorney-client privilege.

Below is an explainer on attorney-client privilege, and how it could feature in the documents investigation.

WHAT IS ATTORNEY-CLIENT PRIVILEGE?

Attorney-client privilege is a long-standing doctrine in U.S. law that permits people to keep communications with their lawyers private.

Lawyers cite the privilege in declining to share information about their conversations with clients. However, it only applies to legal advice and not clients' other conversations with attorneys, said Peter Joy, a professor at the Washington University School of Law in St. Louis.

HAVE THE MATERIALS ALREADY BEEN REVIEWED FOR PRIVILEGE?

The Justice Department said it created a separate "filter" team to view the seized documents and set them aside if they might be privileged.

In a court filing late Tuesday, the Justice Department said that team found "only a limited subset" of records that might be privileged. It said it is prepared to take steps such as asking the judge who approved the search warrant to decide whether the privilege applies.

The team could also seek to negotiate with Trump and ask the court to make a decision on any documents they can't agree on, the Justice Department said.

WHAT IS A SPECIAL MASTER? WHY DOES TRUMP WANT ONE?

In federal court, a special master is a neutral third party appointed to assist a judge.

Here, Trump is arguing that an independent party is needed to determine whether any of the materials seized by federal agents are covered by the privilege and should be returned.

Special masters were previously approved to conduct such reviews after federal searches of the homes and offices of two former Trump lawyers, Michael Cohen and Rudolph Giuliani.

In addition to arguing that Trump has no standing to ask for a special master, Justice Department lawyers said that appointing one would impede their ongoing criminal investigation.

The appointment would be like "throwing sand in the gears" of the probe, as Trump may be seeking to slow it down, Joy said.

WHAT HAPPENS IF INVESTIGATORS SEE PRIVILEGED MATERIALS?

If the team investigating Trump saw any privileged information, those materials could not be used during the rest of its investigation or in any case the federal government seeks to bring, according to Daniel Richman, a law professor at Columbia University.

If federal investigators saw many privileged records, it could result in the current investigative team being disqualified and a new one being brought in, Joy said.

Such a revelation would also fuel Trump claims of a biased investigation.

ARE THERE ANY EXCEPTIONS TO THE PRIVILEGE?

There are select instances in which privileged information can be shared with federal agents. One is if the attorney-client communication is evidence that a future crime will be committed, according to Brandon Fox, a former federal prosecutor. That would include any attempt to cover up a crime, Joy added.

Government lawyers would have to successfully prove to a federal judge a connection between the privileged documents and an alleged crime, Joy said.