user_none, thanks very much for answering my questions so thoroughly.
KevinH, at least in the PMLs generated by Calibre from the PDBs that I have, as viewed in eBook Studio, it is indeed just the two linebreaks between paragraphs. I'm not sure if the centering of the paragraph preceding the two linebreaks has any relevance. Again, when I open the original PDBs in an older version of eReader, these breaks appear as three asterisks. (Haven't you noticed these scene breaks in PDBs? I'm pretty sure they're in the vast majority of the many PDBs I have.) In the final version of eReader, they appear simply as extra space between paragraphs.
I just saw user_none's latest post.
user_none, I read somewhere that if you download a Barnes & Noble e-book using a computer, it's in eReader format. If you use a Nook, it's an EPUB. I'm not sure if this is true, but I thought it was interesting. And wow, you're already making the necessary modifications to support these SSBs!!