[jackie_w]

I know nothing about Kindle apnx files, but I often see html markup something like
<a id="p100" /> or <a id="pviii"></a> in retail epubs. The markup is not visible when reading.

There is also occasionally an xml page-map file in epubs. I know very little about them other than I had to remove them when I was reading on an old Sony device because they caused me problems.

Hopefully someone with better knowledge can add more.

Edit: An epub page-map looks something like this

<?xml version='1.0' encoding='utf-8'?>
<page-map xmlns="http://www.idpf.org/2007/opf">
  <page href="OEBPS/copyright.html#piii" name="piii"/>
  <page href="OEBPS/copyright.html#piv" name="piv"/>
  <page href="OEBPS/preface001.html#pi" name="pi"/>
  <page href="OEBPS/ad-card.html#pii" name="pii"/>
  <page href="OEBPS/dedication.html#pv" name="pv"/>
  <page href="OEBPS/acknowledgements.html#pvi" name="pvi"/>
  <page href="OEBPS/acknowledgements.html#pvii" name="pvii"/>
  <page href="OEBPS/part001.html#pviii" name="pviii"/>
  <page href="OEBPS/part001.html#p1" name="p1"/>
  <page href="OEBPS/chapter003.html#p2" name="p2"/>
  <page href="OEBPS/chapter003.html#p3" name="p3"/>
  <page href="OEBPS/chapter003.html#p4" name="p4"/>
etc ... etc ...
</page-map>

and is referenced if the <spine> tag of the opf file, e.g. <spine toc="ncx" page-map="page-map">