Here's the a2x man page's description of --fop.
--fop
Use FOP to generate PDFs. The default behavior is to use dblatex(1). The --fop
option is implicit if this option is used.
The last sentence about the option being implicit is confusing to me. This option implicitly enables itself? Is one of the option names here wrong? Looking at the a2x.py code, it looks like --fopt-opts implicitly enables --fop, so that sentence should be moved to the description for --fop-opts.