This parameter takes "all, any, phrase". However at the end they all perform the search for "like" not exact match.
If you want to do exact match you need to modify the /include/search.php and change the query from
PHP Code:
$condition[] = $sql_tbl['products_lng_current'] . '.' . $ssf . ' LIKE \'%' . $data['substring'] . '%\'';
to
PHP Code:
$condition[] = $sql_tbl['products_lng_current'] . '.' . $ssf . ' = \'' . $data['substring'] . '\'';
Just a sample code - yours may be different