The labels on these buttons, FO, F, I, P, for Flash Override, Flash, Immediate, and Priority, directly reflected the nuclear C2 scheme at the time.
immediate (adj): Occurring at once; happening without delay
If a bunch of paratroopers just landed outside my facility I need immediate assistance.
If what look like incoming nuclear missiles are spotted that needs to override a call about a bunch of paratroopers.
It's sort of like "our amplifiers go to 11."