According to Barr’s March 29 letter to Congress, there are four categories of information that must not be made public: grand jury matters, compromising intelligence, ongoing investigations, and the privacy and reputation of third parties.

While much of this information is typical for redactions, Barr will have to balance removing the necessary information with maintaining the public’s trust. If he redacts a significant amount, those suspicious of President Donald Trump might believe Barr is trying to hide potentially damaging information.

“I don’t think you could possibly compare anything ever in the history of American jurisprudence to this in terms of what the guidelines are for what should be redacted and what shouldn’t be,” said criminal defense attorney Jeffrey Chabrowe, formerly a prosecutor at the Manhattan District Attorney’s Office.

The grand jury information must remain secret under the Federal Rule of Criminal Procedure 6(e), which places restrictions on the disclosure of matters occurring before a grand jury. Congress, while itself not an exception to the rule, can still obtain the documents in select situations for legislative purposes. The public, however, is not privy to this information.

Barr’s second category of confidential information—”material the intelligence community identifies as potentially compromising sensitive sources and methods”—is more vague. Without seeing the Mueller report, it’s difficult to glean what kind of information this could entail. Being that Mueller was investigating the Russian interference in the 2016 election, however, it’s possible there are elements to the report the intelligence community would want kept secret.

The ongoing investigations could be the largest category leading to redactions, said Chabrowe. As Mueller wrapped up his probe, he referred countless investigations to other offices: the United States Attorney’s Offices for the Southern District of New York, the Eastern District of Virginia, and the District of Columbia have each taken over various investigations or sentencing.

In terms of redacting this information, “it becomes more beneficial for them to acknowledge that there are many more ongoing investigations,” argued Chabrowe.

“The more so they acknowledge that, the more so that they can redact certain things as being pertinent to an ongoing investigation,” he continued. “If it’s heavily redacted and that’s what’s going to be published, then Trump is in a good position to be like, ‘See, there’s nothing here.'”

Chabrowe added that there may be investigations the public doesn’t even know about, which will likely be the most heavily redacted. Regardless, he says, Barr will be “given a lot of leeway” in this area: if anyone later accuses him of over-redacting to protect the president, Barr can justifiably argue he was attempting to protect ongoing investigations.

Barr’s final category for redaction is “information that would unduly infringe on the personal privacy and reputational interests of peripheral third parties.” This could refer to names and identifying attributes of players who were a part of the investigation in some way, but were not themselves involved with any crimes.

Barr’s brief summary of the investigation for Congress says Mueller issued more than 2,800 subpoenas, executed almost 500 search warrants, and interviewed about 500 witnesses. The report itself is nearly 400 pages long, and according to Barr, it lays out Mueller’s “findings, his analysis, and the reasons for his conclusions.”

Whatever he eventually releases to the public, Chabrowe said, “It has to be something intelligible where when you’re reading it, you can actually read it from beginning to end.” He referenced a time he received a redacted document that was practically “10 black pieces of paper.”

Congress wants the full report, and the public wants as much as possible. According to a recent NPR/PBS NewsHour/Marist poll, 75% of Americans want the full Mueller report released—including 54% of Republicans.

Mueller may have some input on the redactions, but as attorney general, Barr has the final say.