A Appointment Letter is the letter come from U.S. Consulate General when an visa interview has been scheduled or ready to be scheduled. Depends on the country and visa type, the appointment letter can be a email or a letter. Visa applicants must bring the appointment letter to the interview. If the letter is a email, you need to print all context of the email and take it to the interview.
In the letter, a detailed instruction for interview preparation will be listed including medical exam instruction, register for courier service instruction and documents to bring to the interview checklist. If NVC schedules your interview appointment, appointment date and time will also be listed in the email or letter. Visa applicants must read all instruction carefully as each individual might receive different pre-interview instructions based on their background and application categories.
Note that medical exam appointment can only be scheduled after you receive your appointment letter.
Example of a K-1 visa appointment letter