Nikon D810 - Manual (Page 221)

197
A
Optional Flash Units
The flash compensation selected with the optional flash unit is added
to the flash compensation selected with the camera.
A See Also
For information on choosing the size of the increments available for
flash compensation, see Custom Setting b3 (Exp./flash comp. step
value, 0 315). For information on choosing how flash and exposure
compensation combine, see Custom Setting e4 (Exposure comp. for
flash, 0 338). For information on automatically varying flash level
over a series of shots, see page 133.