<fw> (forme work) contains a running head (e.g. a header, footer), catchword, or
similar material appearing on the current page. [11.6 Headers, Footers, and Similar Matter]
Where running heads are consistent throughout a chapter or
section, it is usually more convenient to relate them to the
chapter or section, e.g. by use of the rend attribute.
The fw element is intended for cases where the running
head changes from page to page, or where details of page layout
and the internal structure of the running heads are of paramount
importance.