The origin of the Promega is located at the front-top-left corner of the printer, where the three red lines intersect. If the printer moves to X0, Y0 and Z0, or (0,0,0) it will end up in this corner. The position of the nozzle of the printer is measured from this point. As mentioned before, the printer moves in the X and Y direction with the a coreXY belt system, which we will go over later. The Z-axis works differently, instead of moving the nozzle away from the bed, the Promega moves the bed away from the nozzle. The nozzle is always located on a single plane which it can be moved across by the coreXY system. The Z-axis distance value increases as you move the bed down, away from the nozzle, and the distance decreases as you move the bed up, toward the nozzle. This might be counter-intuitive at first, as a normal oriented cartesian coordinate frame will have the Z-axis pointing up, but you will get used to it quickly.