Mailing notations are instructions to the postal service and mail rooms on how to process a letter, such as "confidential," "special delivery," "certified mail" and "airmail." Formal business letters include mailing notations on the actual letter and on the envelope, explains About.com.
When typing or printing formal business letters, mailing notations are made in all capital letters. The mailing notation is entered on the letter four lines below the reference line, or the date if a reference line is not used. The typical order for a business letter is the return address, the date, a reference line explaining what the letter is in response to, four spaces and then any mailing notations. The return address is unnecessary when using letterhead stationery. Enter multiple mailing notations each on a separate line without spaces between them.
When addressing an envelope, mailing notations are entered directly underneath the stamp or postage. The mailing notation is written in block-style capital letters. The U.S. Postal Service prefers all information entered on an envelope to be written in block-style capitals to facilitate machine processing. If the notation provides directions to the postal service, such as "express mail," the post office supplements your notation with an official stamp when your letter is processed. If the notation is for a mail room clerk or secretary, such as "confidential," the post office will disregard the notation, but mail room personnel and administrative assistants use the notation to deliver the mail properly.