If you select the objects by dragging a bounding box, the first object in the order stack is used as the Stationary Object. If you select the objects by clicking them while holding Shift, the first object selected is used as the Stationary Object.
In the example below, the yellow square is used as the Stationary Object for the alignment.
Original Objects |
Left Alignment |
Right Alignment |
Horizontal Centers |
Both Centers |
Top Alignment |
Bottom Alignment |
Vertical Center |