I had some help from Hal himself: the thread starts with him complaining about over-chaptering in a manuscript he's critiqueing, and ends with Official Terminology for describing his “formalist shenanigans”:
Hal would treat the “Errata” sections as chapter appendices. I've represented them in the html as a header level below Chapter and above Canto; I think this is fair to his intention.
The printed and ebook editions also contain centered stars, which look like they convey yet another level of hierarchical structure. In fact, however, these are simply divisions between verses: when a verse boundary coincides with a page boundary, there is no visible whitespace division so the publishers added the stars to indicate the same structural feature. These have been taken over into the ebooks, where they are useless (since they generally no longer coincide with the page boundaries) and misleading (for anyone as obsessively attentive to structural cues as ourselves). I have removed them from the html files.