Final answer:
The initials of the letter writer followed by a colon or slash and the typist's initials are known as reference initials, indicating who wrote and typed the letter.
Step-by-step explanation:
The answer to the student's question about the use of initials in a business letter format is that the letter writer's initials followed by a colon or slash and the typist's initials are known as the reference initials. These initials are included as a form of notation that indicates who wrote the letter and who typed it. This is particularly useful in office environments where the person who wrote the letter is not the same as the person who typed it.
Typically, the letter writer's initials are in uppercase letters and the typist's initials are in lowercase. This notation is placed a couple of lines below the signature block or the typed name of the sender. For example, if John Doe wrote the letter and Jane Smith typed it, the reference initials would appear as JD/js, indicating their respective contributions to the document.