Interview-based letters can be sent from the Interview manager.
Letters to be printed for interviews should have a Data source of Interviews. This allows you to include all types of interview-based information like the start time, interview type, location, and other properties. VSys comes with one built-in letter of this type, and you can copy and customize that to meet your specific needs. For example, you can have one template for regular interviews, one for "volunteer satisfaction", and one for screening results that you send after the interview.