Nikon D6 Reference Manual page 267

A
Hi 0.3–Hi 5
A setting of [Hi 0.3] corresponds to an ISO sensitivity approximately
0.3 EV higher than ISO 102400 (ISO 128000) and [Hi 5.0] to an ISO
sensitivity approximately 5 EV higher (ISO 3280000). Note that pictures
taken at these settings are particularly prone to "noise" in the form of
randomly-spaced bright pixels, fog, or lines.
A
Lo 0.3–Lo 1
[Lo 0.3] corresponds to an ISO sensitivity about 0.3 EV below ISO 100
(equivalent to ISO 80). [Lo 1.0] is roughly 1 EV below ISO 100 (ISO 50). Use
for larger apertures or slower shutter speeds when lighting is bright.
Highlights may be overexposed. In most cases, ISO sensitivities of ISO
[100] or above are recommended.
Adjusting the Camera's Sensitivity to Light (ISO Sensitivity)
215