When the report is written in the letter of transmittal, it is known as letter report. It does not include title fly, title page, authorization letter, and table of contents. Letter reports move to the people outside the organization. It may or may not have subject line. It may serve to inform or to recommend.
According to Murphy and Peck, letter reports don’t include introduction and summary, conclusion, or recommendations. While other authors like Raymond Dumont, John Lamnon, Norman Sigband, David Bateman, and many more favor the inclusion of the headings of introduction and recommendations.
A letter report is less formal than the formal report and more formal than the memorandum report.