Nikon D750 User Manual page 217

At values other than ±0.0, a Y icon will be displayed in the
control panel and viewfinder after you release the M (Y) button.
The current value for flash compensation can be confirmed by
pressing the M (Y) button.
Normal flash output can be restored by setting flash
compensation to ±0.0. Flash compensation is not reset when
the camera is turned off.
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 b2 (EV steps for exposure
cntrl, 0 333). For information on choosing how flash and exposure
compensation combine, see Custom Setting e4 (Exposure comp. for
flash, 0 353). For information on automatically varying flash level
over a series of shots, see page 202.
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