It's easiest to think of the options as describing which way the list window will grow if someone new logs in. For example, if the Horizontally: option is set to Left, the list window's left edge will move when people log in and out, with the right edge remaining stationary; so, when someone logs in, the window will grow to the left. If it's set to Center, the window will "grow to the center"; that is, the window will stay horizontally centered on its current position. If it's set to Right, the window will grow to the right.
The Vertically: options are analagous to the Horizontally: ones.