My Nissin Di866 II flashgun supports operation in both "slave flash" and "master flash" mode. The former I know needs to be used with a Nikon DSLR's "Flash Commmander" mode.
What's the latter mode? Where is the difference?
Slave flashes are off-camera flashes which are triggered by a master flash (or a commander like the SU-800 or the built-in commander in many Nikon bodies).
So the master flash mode allows the Nissin to control the off camera flashes that are in slave mode.
This can be done in two ways:
This nomenclature is for using multiple flashes simultaneously. In such setup, one flash is the master and all other ones are slaves: