Nikon D780 DSLR Camera (Body Only) - Manual (Page 223)

177 Shooting Settings
A Mode M
In mode M , exposure compensation affects only the exposure indicator;
shutter speed and aperture do not change.
A Using a Flash
When a flash is used, exposure compensation affects both flash level and
exposure, altering the brightness of both the main subject and the
background. Custom Setting e3 [
Exposure comp. for flash ] can be used
to restrict the effects of exposure compensation to the background only.