Difference between 'DEST' & '--backup-dir='?

[gnu69 at free.fr]

Hello all,

I have a question.
The man page of rsync:

SYNOPSIS
rsync [OPTION]... SRC [SRC]... DEST


RÉSUMÉ DES OPTIONS
--backup-dir            effectue des sauvegardes dans ce répertoire


What is the difference between 'DEST' and the rsync option '--backup-dir='?


Thank you very much.


paco