If there is more than one recipient in the recipient list of a letter, it’s a serial letter.
You can create a serial letter based on a template that contains a standard text for all recipients. The word processer generates as many letters as there are correspondence recipients. The serial letters can be printed out immediately or be saved first. You can also choose whether only a single letter of the series is stored as a sample for the text, or all of them should be saved in the database. See also the Note on sample correspondences for serial letters.