Here's why some sensitive documents are redacted

(CNN)After Robert Mueller spent 22 months investigating, the Justice Department released a redacted version of his nearly 400-page report.

Here's why the government redacts documents and what you can expect to be missing from the Mueller report.

Redactions are fairly common

Redacting means editing a document to delete or mask information that has been deemed as privileged or confidential, says Lisa Gilbert, vice president of legislative affairs at Public Citizen.
    "It's a pretty common practice in legal documents," Gilbert said.
    Typically, personal data such as someone's social security number is removed from public legal documents to ensure privacy. Other sensitive information can include medical history and trade secrets.

    The redactions in the Mueller report

    US Attorney General William Barr held a highly criticized press conference before the release of the redacted Mueller report on Thursday.
    Attorney General William Barr said the report will include four types of redactions.
    Grand jury information: Federal law says that matters before grand juries, which are secret panels that decide whether someone should be charged with a crime, should remain private.
    Classified information: This is material such as sources in the intelligence community or methods of intelligence collection. If released, this could compromise future intelligence gathering.
    Material about ongoing investigations: This includes information that is related to or could influence ongoing cases. During Mueller's investigation, several spinoff cases arose and are now being investigated by federal prosecutors.
      Material affecting peripheral third parties: This is a vague category that could encompass information related to anyone from longtime Trump ally Roger Stone to former Trump campaign manager Paul Manafort -- basically anyone who has been in the mix as Mueller's team investigated collusion and obstruction.
      Some of the redactions will be due to the gag order in Stone's ongoing case, federal prosecutors said in a court filing. Prosecutors say they are doing so to avoid potential prejudice to a jury, since Stone has pleaded not guilty and is headed to trial.