This pair of names is the interscript file name convention. If the prefix is empty, it is replaced by the absolute pathname of the current directory in native operating system format, including a trailing directory separator. Then the Unix filename is converted to a native operating system filename, and appended to the prefix.
Commands such as 'include_file' which refer to sources supply the directory of the current file as the prefix so that the argument filename is relative to the location of the including input file. All such arguments must be relative filenames in Unix format for this reason.
Note that the command line provides the '--source-prefix=' option for the same reason: a filename given on the command line as a master source file must be given as a relative filename in Unix format, even on non-Unix systems. The source-prefix option permits the native operating system name of the directory containing the file to be specified.