Guidelines, principles and structure
6. Letter of transmittal
If the document is delivered to external organization (i.e. operator, city office, city council, media), you should also use a letter of transmittal, which describes the general nature of the research, introduces the report, defines the persons for whom the report is designed and highlights the crucial/most important elements of the report.
The principal purpose of a letter of transmittal is to orient the reader to the document and to build a positive image of the report.