It's not a simple answer and they can't all be bunched together. Some are CPS1 pals that have been dumped but not added to mame yet. Some are MCUs which have been secured but not decapped yet. Some are random pals that would just be dumped for documentation and not actually hooked up (only referenced) thus not having any positive change on emulation accuracy. Some are probably needed MCUs but who knows if they would ever be decapped if you got them. We still have a pile already secured and not done yet so...Guru has a list of still-needed MCUs.