Not sure, but fairly confident that is black tank flush. I've never seen one that flushes both black and grey tanks.
When draining your black tank (gate valve open), hook garden hose to it and let water run for 5 minutes or so. If installed properly, it should help dislodge solids that remain in the tank. Mine works real well.
The first time you use it, make sure you keep an eye under the trailer for water running out of the underbelly (just in case it leaks).
2010 Keystone Outback Sydney 5'er
2012 Chevy Silverado 1 ton, LTZ, Standard bed, Crew, 6.0 Gasser, 4:10 gears