dbhtml_dir — Specifies a directory name in which to write files
<?dbhtml dir="path"?>When chunking output, use the <?dbhtml dir?> PI
      as a child of a chunk source to cause the output of that
      chunk to be written to the specified directory; also, use it
      as a child of a mediaobject to specify a
      directory into which any long-description files for that
      mediaobject will be written.
The output directory specification is inherited by all
chunks of the descendants of the element.  If descendants need
to go to a different directory, then add another 
<?dbhtml dir?> processing
instruction as a child of the source element
for that chunk, and specify the path relative to the
ancestor path.
For example, to put most chunk files into 
shared
but one chapter into 
exception
at the same level, use:
<book>
  <?dbhtml dir="shared"?>
  ...
  <chapter>
    <?dbhtml dir="../exception"?>
  </chapter>
</book>