To use a tar file other than /dev/rst0, use the f keyletter, as in

tar cf directory.tar directory

The tar file name must be the word after the keyletter word.  To send the
tar file to the standard output, use "-" for the file name, as in

tar cf - directory

Run ``man tar'' for other examples of how to use tar and a complete list of
the amazing array of options.

(Sheesh, and after all the effort I went to writing the manual page.)