In the Redacted Mueller report, four reasons for redaction are given:
- Harm to Ongoing Matter
- Personal Privacy
- Investigative Technique
- Grand Jury
What is the official explanation, with citations, for these four reasons?
Refers to ongoing investigations (possibly classified, depending on the subject). Barr addressed this directly
"Most of the redactions were compelled by the need to prevent harm to ongoing matters and to comply with court orders prohibiting the public disclosure of information bearing upon ongoing investigations and criminal cases, such as the IRA case and the Roger Stone case," Barr said at Thursday's press conference prior to the report's release.
Pretty self-explanatory. Someone provided information that wasn't illegal, but personally embarrassing or damaging if it became public. To help ensure people help the government like this in the future, promises were made to keep it secret. This is not a legal category
The [other reasons for redaction] have a pretty solid legal basis. This one is is much more subjective, and potentially much broader. You can imagine circumstances where it would be appropriate to accept that kind of restriction, and other circumstances where it could be abused.
Mostly covers methods used to gain information they don't want exposed. Like spying
[I]t’s highly likely the US government had to use sensitive spying methods — like, say, an undercover agent or top-secret surveillance technology — to investigate. Naturally, the US wants to keep that kind of stuff, known as “sources and methods,” under wraps so that spies keep their cover and other governments don’t figure out how America keeps tabs on them.
“If we publish the wrong stuff, people could get killed,” Wright said.
Grand Jury testimony is, by law, supposed to be secret.
Traditionally, the grand jury has conducted its work in secret. Secrecy prevents those under scrutiny from fleeing or importuning the grand jurors, encourages full disclosure by witnesses, and protects the innocent from unwarranted prosecution, among other things. The long-established rule of grand jury secrecy is enshrined in Federal Rule of Criminal Procedure 6(e), which provides that government attorneys and the jurors themselves, among others, “must not disclose a matter occurring before the grand jury.” Accordingly, as a general matter, persons and entities external to the grand jury process are precluded from obtaining transcripts of grand jury testimony or other documents or information that would reveal what took place in the proceedings, even if the grand jury has concluded its work and even if the information is sought pursuant to otherwise-valid legal processes.
There's a lengthy section on why that excludes Congress. A more succinct case is this (link moved for clarity in quote)
Does a federal court have the authority to order disclosure of grand-jury materials if the judge decides that the interests of justice warrant doing so; or is the judge limited to the exceptions to grand-jury secrecy that are spelled out in Rule 6(e) of the Federal Rules of Criminal Procedure? The D.C. Circuit’s McKeever ruling holds that the text of Rule 6(e) controls. Consequently, judges have no authority to authorize disclosure outside the rule.
This is significant for the Mueller report because Rule 6(e) does not contain an exception to secrecy that would permit disclosure to Congress.