Nikon D780 DSLR Camera (Body Only) - Manual (Page 239)

193 Shooting Settings
When [ On ] is selected for [ Auto ISO sensitivity control ], ISO
AUTO indicators will appear in the control panel and
viewfinder. When these indicators are lit (not flashing), pictures
will be taken at the sensitivity selected for [
ISO sensitivity ].
When sensitivity is altered from the value selected by the user,
the
ISO AUTO indicators will flash and the altered value will be
shown in the displays.