"Rich Text Format" -- (.rtf) refers to a standardized word processor document format that most word processors can read and understand. Time Logger uses this for its invoice templates. It also generates new invoices based on the invoice template and saves them as .rtf.
NOTE: Whenever you edit an invoice template, it must be saved as a .rtf type file. When this is the case, its type is "Rich Text Format" and it has a .rtf extension.