nxml-heading-element-name-regexp

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

"title\\|head"

Documentation:
Regular expression matching the name of elements used as headings.
An XML element is only recognized as a heading if it occurs as or
within the first child of an element that is recognized as a section.
See the variable `nxml-section-element-name-regexp' for more details.

You can customize this variable.