DOCBOOK: confused about callouts

["Robert P. J. Day" <rpjday@mindspring.com>]

  i have a section of a much larger docbook document, with the following
<screen> element and <co> callout marks:

    <screen>
<filename>/etc/passwd</filename>                     <co id="/etc/passwd"/>
<filename>/etc/shadow</filename>                     <co id="/etc/shadow"/>
<filename>/etc/group</filename>                      <co id="/etc/group"/>

<filename>/etc/profile</filename>                    <co id="/etc/profile"/>
  <filename>/etc/profile.d/*.sh</filename>           <co id="/etc/profile.d"/>

<filename>~/.bash_profile</filename>                 <co id="bash_profile"/>
<filename>~/.bashrc</filename>                       <co id="bashrc"/>
  <filename>/etc/bashrc</filename>                   <co id="/etc/bashrc"/>
    </screen>
    <calloutlist>

     <callout arearefs="/etc/passwd">
     <para><filename>/etc/passwd</filename> defines, among other
      things, the username, numeric UID and GID of the user, the
      home directory and the login shell.   For security reasons,
      the encrypted password is no longer stored in this file.</para>
     </callout>

     <callout arearefs="/etc/shadow">
     <para><filename>/etc/shadow</filename> represents a more secure
      place to store things like the encrypted password, password aging
      information and more.  (This is the only file in this list of files
      that requires root privilege to display.)</para>
     </callout>

     ... etc etc ...
    </calloutlist>

the HTML for this section can be viewed at
www.linux-migration.org/ch02s03.html, for which i have a
few questions.

1) trivial issue, but viewing with mozilla and galeon, there is
   an apparent line feed between the numbers and the assoc. text
   in the callout list itself.  using IE, however, it looks
   correct -- no mysterious line feed.

2) a <co> element has both an id= and a label= attribute.
   clearly, i use the id attribute for the linking -- i'm not
   sure what the label attribute is for.  the online TDG states
   that it "specifies an identifying number or string that may
   be used in presentation."  

   any hints as to what the label attribute is used for?

3) the online TDG also documents that a <callout> has the
   arearefs attribute, which is exactly what i use above to 
   refer back to the appropriate <co>.

   however, the documentation for the <callout> element states
   that the "arearefs" attribute "must point to one or more Areas
   or AreaSets".  but mine don't -- they refer to simple
   <co> elements.  

   am i misreading something here?

rday

p.s.  i'm sure i'll have more later once this confusion is
cleared up.