The backmatter element is stocked by the citation, index and note elements.
Within your document you may refer to books and articles using the
citation element, and these are collected together in the backmatter
element. The DTD is quite generic, but in this application, the
bibliography is implemented as a BibTeX database, so that the
backmatter element's `bibliography' attribute names a BibTeX.bib file (without the .bib extension), and the
citation element's content is a BibTeX entry key. Note that the
BibTeX processing of your document is done within the context of a
temporary work directory (see Section 6.2), so you might have
to specify BibTeX's BIBINPUTS environment variable if
BibTeX is to find your bibliography database. There is no way (at
present) to specify the bibliography formatting style - the SGML set
uses a customised bibliography style which, incidentally, supports a
URL field in bibliography entries. See the BibTeX documentation in,
for example, [lamport] for further details.
You can include notes in your document with the note element. At present, the notes are presented as footnotes in the paper version of your document and collected together as endnotes in the online version.
Although the indexing support is defined in the DTD, there is not
yet any application support for it. It is likely to use the
makeindex system associated with LaTeX.