BookNames table in Scripture Reference settings is usually a source of data for running header on the printed page. But for various reasons typesetter might be asked to place in running header something different from \toc1 (long name), \toc2 (short name) or \toc3 (abbreviation).
Custom header content codes allows typesetter to use not only BookNames table but also content of the
\h tag in USFM file. E.g., code
<bc.r.s> will place book name and chapter range, and the short name from BookNames table will be used (“.s” means “use short names from BookNames.xml”). But code
<bc.r.h> means “use
\h content for the book name in the running header”. This flexibility allows more diversity for the printed books without changing the project settings. More details about custom header content codes are available in the PA User Manual.