Delta peak on the 959 is the millivolts per cell down from the maximum voltage the pack achieves during charging that will stop the charge.

If you are charging 6 cells in a pack with the delta peak set for 5mv/cell, when the pack drops 30mv below the highest voltage it reached during the charge, the 959 will consider the cells charged and stop the current to the pack. They are saying 'delta peak' as a short way of saying 'change in value from peak', using the word delta in it's mathmatical definition.

5mv/cell is the usual normal setting for nimh cells, 10 to 15 mv/cell for nicad.

New packs may false peak when charging at low currents for the first few times when set at 5mv, so some set to 8mv, for nimh, then set back to 5mv after the pack gets to feeling better in a few cycles.

If you set it to zero on a charger, then it's just 'peak' since the delta is zero.