frtbak.sty is a special copy of the usfm.sty sheet where many of the "\OccursUnder" have been removed. For example, in the usfm.sty \p occurs under c. However, in the frtbak.sty it can occur anywhere. This is why when you remove the frtbak.sty you get red markers in the extra material books. You can handle this by simply adding a \c at the beginning of those books, but it is probably better to use the frtbak.sty