For Round and N-Sided stock, the axis that corresponds to the length of the stock is specified as the Axis.
Z axis:
Select Axis: Z for all turning, turn/mill parts or milling parts where you want the XY plane to correspond to the face of the part.
Y axis:
Select Axis: Y for all 4th axis wrapping parts where the Y axis is the wrapping axis.
X axis:
Select Axis: X for all 4th axis wrapping parts where the X axis is the wrapping axis.