For special element units file and target (see Output Element Units), the function reference is:
($target, $file) special_element_target_file_name ($converter, \%element, $default_target, $file_name) ¶$converter is a converter object. \%element is the Texinfo element corresponding to the special element unit. $default_target is the target that has been already set, and $file_name is the file name that has been already set. The function should return the $target and $file.
To determine the variety of the special element processed, the extra
hash special_element_variety key can be used.
See Table 15.1.