the editing of exempt material from information held by public bodies." Secure redacting is more complicated with computer files. Word processing formats may save a revision history of the edited text that still contains the redacted text. In some file formats, unused portions of memory are saved that may still contain fragments of previous versions of the text. Where text is redacted, in Portable Document (PDF) or word processor formats, by overlaying graphical elements (usually black rectangles) over text, the original text remains in the file and can be uncovered by simply deleting the overlaying graphics. Effective redaction of electronic documents requires the removal of all relevant text and image data from the document file. This process, internally complex, can be carried out very easily by a user with the aid of "redaction" functions in software for editing PDF or other files. Redaction may administratively require marking of the redacted area with the reason that the content is being restricted. US government documents released under the Freedom of Information Act are marked with exemption codes that denote the reason why the content has been withheld. The US National Security Agency (NSA) published a guidance document which provides instructions for redacting PDF files. Printed matter Printed documents which contain classified or sensitive information frequently contain a great deal of information which is less sensitive. There may be a need to release the less sensitive portions to uncleared personnel. The printed document will consequently be sanitized to obscure or remove the sensitive information. Maps have also been redacted for the same reason, with highly sensitive areas covered with a slip of white paper. In some cases, sanitizing a classified document removes enough information to reduce the classification from a higher level to a lower one. For example, raw intelligence reports may contain highly classified information such as the identities of spies, that is removed before the reports are distributed outside the intelligence agency: the initial report may be classified as Top Secret while the sanitized report may be classified as Secret. In other cases, such as the NSA report on the USS Liberty incident (right), the report may be sanitized to remove all sensitive data, so that the report may be released to the general public. As is seen in the USS Liberty report, paper documents are usually sanitized by covering the classified and sensitive portions before photocopying the document. Computer media