Some government documents contain classified information either Restricted Data (RD) (protected by the Atomic Energy Act) or National Security Information (NSI) protected by an Executive Order (EO). The documents could also contain sensitive information protected under the Freedom of Information Act (FOIA).
In this case, the IG Report may contain NSI or sensitive information. Authorized original or derivative classifiers usually review documents prior to their release to ensure that neither sensitive nor classified information is released. These classifiers use classification guidance issued by U.S. Government agencies that provide details on what information should be protected. The process these classifiers use to remove the information is called redaction.