...
For vertical distribution, Middle means equal spacing between reference points, and Space means equal spacing between object bounding boxes. For horizontal distribution, Center means equal spacing between reference points, and Distance means equal spacing between object bounding boxes.
In this example, Object 2 is the highest, followed by 1, 3, and 4.
After vertical distribution, the highest and lowest objects (Objects 2 and 4) remain in place. Objects 1 and 3 are moved so that top-to-bottom order remains the same: 2, 1, 3, 4. 