They're not supposed to contain data, but some parsers will allow you to access what's written into comments. And so, of course, someone made use of that and I had to extract what was encoded basically like that:

<!--
    Host: toaster,
    Location: moon,
-->
<data>Actual XML follows...</data>

My best guess is that they added this data into comments rather than child nodes or attributes, because they were worried some of the programs using this XML would not be able to handle an extension of the format.