Nikon D750 DSLR Camera (Body Only) - Manual (Page 214)

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The flash control mode for the built-in flash can be selected using
Custom Setting e3 (
Flash cntrl for built-in flash , 0 347). The
information display shows the flash control mode for the built-in flash
as follows:
Flash sync Auto FP ( 0 345)
i-TTL —
Manual —
Repeating flash —
Commander mode