For user notification emails...
In the
usernotify subroutine in
TTXTicket.pm, find these lines...
$subject = TTXDictionary::translate('Ticket').' '.$ticket->{id};
$subject .= ", ".$ticket->{subject} if $ticket->{subject} ne undef;
Somewhere into the above, you would insert the following for custom variable "0"... (change to the number for your variable)
$ticket->{"c0"}
If you can't figure out the PERL, post here what you want the subject line to say.
And for the operator notifications, find the $subject line in the
opernotify subroutine and do the same.