The black tank flush is for cleaning out the black tank. At the dump station they somtimes have a hose that you hook up to the flush. There is a sprayer in the tank that washes the inside of the tank.
I have the vibe 6501 and have used it a few times.
It is labeled black tank flush. If you look on line you will see pictures that some dealers have taken that will show it. The only option on the 6500 series is the TV. The other options are mandatory, not sure how that makes them an option.