-d ${user}@${nexthop} or -d ${recipient}?

What's the differency between these two lines;

1- flags=DORhu user=vmail:vmail argv=/usr/local/libexec/dovecot/dovecot-lda -f ${sender} -r ${recipient} -d ${user}@${nexthop} 2- flags=DRhu user=vmail:vmail argv=/usr/local/libexec/dovecot/dovecot-lda -f ${sender} -d ${recipient}

and which one should I use, based on a setup like; Dovecot, Postfix, Postfixadmin, ManageSieve and virtual users hosted in SQL?

Thanks in advance.