I think I can make sense of this. It turns out that in some design situations elements are located in x: y ( horizontal and vertical ) coordinates with keywords for the endpoints. Those words are "left, center, right" for x, "up, center, down" for y; which leaves 'center-center' for absolute centering, horizontal and vertical. As a way to avoid confusion you usually use the "up, middle, down" variant, but most of the time it is unnecessary.
