The same generic box is used on a number of different DROs. On some, the buttons are marked ADD and SUB, or + and -, or with arrows : they add or subtract 0.01mm to the scale value. Others label the buttons ABS and HOLD or SET. HOLD locks the display value and ABS switches between relative and absolute measurements.
On my WM18 the arrow buttons don't do anything and they're not mentioned in the manual. I think it's because the mill is fitted with the simplest possible unit. Sufficient for the spindle that the gauge only does ON/OFF, ZERO, and metric/imperial. I've not missed the other functions at all, though I suppose they might be handy if the mill were used as a sort of height gauge. I don't think it's a fault.
Dave