Thank you for the reply.
I just did a simple experiment with just XML files.
In opf file, I defined:
<item id="titlepage" href="title.xml" media-type="application/xhtml+xml"/>
<item id="note" href="note.xml" media-type="application/xhtml+xml"/>
<item id="kml" href="kml_sample.kml" media-type="application/google+kml" fallback="titlepage"/>
In note.xml, I added a link like this:
<p><a href="kml_sample.kml">KML</a></p>
From my understanding, if fallback is supported, when the KML link is clicked, reader app should jump to title.xml. DE displayed KML as a link, but when I clicked, nothing happened.