nxml-section-element-name-regexp

nxml-section-element-name-regexp is a variable defined in `nxml-outln.el'.
Its value is

"article\\|\\(sub\\)*section\\|chapter\\|div\\|appendix\\|part\\|preface\\|reference\\|simplesect\\|bibliography\\|bibliodiv\\|glossary\\|glossdiv"

Documentation:
Regular expression matching the name of elements used as sections.
An XML element is treated as a section if:

- its local name (that is, the name without the prefix) matches
this regexp;

- either its first child element or a descendant of that first child
element has a local name matching the variable
`nxml-heading-element-name-regexp'; and

- its start-tag occurs at the beginning of a line (possibly indented).

You can customize this variable.