Any occurrences of $H are replaced with the portion of the host/domain specification that was not matched by the rule. Any occurrences of $D are replaced by the portion of the host/domain specification that was matched by the rewrite rule. The $nH and $nD characters are variants that preserve the normal $H or $D portion from the nth leftmost part starting counting from 0. That is, $nH and $nD omit the leftmost n parts (starting counting from 1) of what would normally be a $H or $D, substitution, respectively. In particular, $0H is equivalent to $H and $0D is equivalent to $D.
For example, assume the address email@example.com matches the following rewrite rule:
The resulting address is firstname.lastname@example.org with TCP-DAEMON used as the outgoing channel. Here where $D would have substituted in the entire domain that matched, host.siroe.com, the $1D instead substitutes in the portions of the match starting from part 1 (part 1 being siroe), so substitutes in siroe.com.
$L substitutes the portion of a domain literal that was not matched by the rewrite rule.