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:
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>