Positive Zero Error = + 0.03 mm. It is written as positive as the zero mark of the thimble has gone beyond (to the right of) the zero mark on the datum line.

Since you turn the thimble towards you to retract the spindle from the anvil, you have 3 divisions less to turn. So this zero error of 0.03 mm has to be subtracted from the final reading.

Negative Zero Error = - 0.04 mm It is written as negative as the zero mark of the thimble is before (to the left of) the zero mark on the datum line.

Since you turn the thimble towards you to retract the spindle from the anvil, you have 4 divisions more to turn. So this zero error of 0.04 mm has to be added to the final reading.