I hope this posts to the right place, if not I apologize.
I wanted to add a class to this div so that I could target the subsidiary asides diferently depending how on how many were active. Here's what I did:
open up sidebar-subsidiary.php, goto line 4 which looks like: <div id="subsidiary"> and change it by copy pasing the following over it:
<?php
$sub_count = 0;
if ( is_sidebar_active('1st-subsidiary-aside'))
$sub_count ++;
if ( is_sidebar_active('2nd-subsidiary-aside'))
$sub_count ++;
if ( is_sidebar_active('3rd-subsidiary-aside'))
$sub_count ++;
?>
<div id="subsidiary" class="subs-<?php echo $sub_count; ?>">
There may be a better way, but this works so I thought I'd share it. I'm currently using it to adjust the width on the sub-aside divs depending on how many there are.
Thanks for reading :)