Nikon D780 Reference Manual page 546

3
C : Adjust flash settings.
•
Choose the balance between
groups A and B.
•
Adjust flash compensation for
groups A and B.
•
Choose a flash control mode and
flash level for the units in group C:
- [M]: Choose the flash level
manually.
- [––]: The units in group C do not
fire.
•
Choose a channel for the master
flash. If the remote flash units
include an SB-500, you must choose
channel 3, but otherwise you can
choose any channel between 1 and
4.
500 Remote Flash Photography