b1 ISO Sensitivity Step Value
EV means „exposure value“; a change of one exposure value means to double or half the exposure. The series of common ISO sensitivities 100, 200, 400, 800, 1600 etc. is using steps of 1 EV. When choosing 1/2 EV in this menu you get everywhere one additional value in-between, when choosing 1/3 EV you get two.
The auto ISO sensitivity control is not affected by this menu and will always use an even finer gradation than 1/3 EV.
Tips: Use 1/2 EV instead of the default. This lets you flip through the ISO values more quickly and the accuracy is still sufficient.