One organizational unit of a book, typically called a chapter, but possibly also called a part, module, section, unit, topic, volume, etc.
The book-part-type attribute can be used to name the published name of the component for text generation or other purposes, for example:
<book-part part-type=“chapter”>
As a hierarchical matter, book parts with the title “Part” typically contain other <book-part>s. Book parts with the title “Chapter” typically contain “sections” subunits that are just heads and text, i.e., they have no back matter such as appendices and references. Consequently, if a division of the hierarchical text (in other words, not something “out of line” such as a sidebar) has formal back matter, then it is a <book-part>. The deciding factor is how much metadata you need (sections have only titles, while book parts have a lot of metadata) and whether or not there is formal back matter.
<!ELEMENT book-part %book-part-model; >
The following, in order:
<book>
<book-meta>...</book-meta>
<book-front>...</book-front>
<body>
<book-part id="bid.2" book-part-type="chapter" book-part-number="1">
<book-part-meta>...</book-part-meta>
<body>
<sec id="bid.3">
<title>History</title>
<p>Initially, GenBank was built and maintained at Los Alamos
National Laboratory (<xref ref-type="kwd" rid="bid.41">LANL</xref>). In the early 1990s, this responsibility was awarded to NCBI through ...</p>
</sec>
<sec id="bid.4">
<title>International Collaboration</title>
<p>In the mid-1990s, the GenBank database became part of the International Nucleotide Sequence Database Collaboration with the EMBL database ...</p>
</sec>
</body>
<back>...
</back>
</book-part>
</body>
</book>
bookpart.ent