E2: Flash Shutter Speed; E3: Exposure Comp. For Flash - Nikon D780 Reference Manual

e2: Flash Shutter Speed

Choose the slowest shutter speed available with a flash in mode
P or A.
Regardless of the setting chosen for e2 [Flash shutter speed],
shutter speeds can be as slow as 30 s in modes S and M or at
flash settings of slow sync, slow rear-curtain sync, or red-eye
reduction with slow sync.

e3: Exposure Comp. for Flash

Choose how the camera adjusts flash level when exposure
compensation is used.
Option
[Entire
YE
frame]
[Background
E
only]
620 Menu Guide > A Custom Settings
G button U A Custom Settings menu
G button U A Custom Settings menu
The camera adjusts both the flash level and
exposure. This changes the exposure for the
entire frame.
Exposure compensation is adjusted for the
background only.
Description