Coordinates are measured from the bottom center of the fixture. The small red box in the preview window indicates the current position of the light source. Changing the X value moves the light source left or right. Selecting Middle positions the light in the center of the fixture, and selecting Minimum or Maximum positions it on the left or right side. If you select Custom you can enter a specific value in the adjacent edit box which is relative to the center position. For example, entering -3 moves the light source 3” left from the center. Changing the Y value moves the light source forward or backward. Selecting Minimum brings the light source all the way forward, and selecting Maximum moves it to the back of the fixture. Changing the Z value moves the light source up or down. Selecting Minimum positions the light source at the bottom of the fixture, while selecting Maximum positions it at the top of the fixture.