<fw> (forme work) contains a running head (e.g. a header, footer), catchword, or similar
material appearing on the current page. [12.6 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.