X moving away from centre line is positive and Z moving towards tailstock end is positive. If you think of the X and the Diameter of the part, X0 would be centre X10 would be 10mm Dia, X20 20mm Dia etc etc.
Normally you set your work coords for Z to zero at the end of the stock and thus all cutting in Z is negative values. It is the same as a mill or a router, normally you set the top of the material as Z0 and all cutting in Z is negative, just think of a lathe as a mill lying down on its back and it should help.
Hood