There are three Center commands: Vertically, Horizontally and Both. When you select a Center command, the selected polygon(s) are moved to the center of the substrate. If a single polygon is selected, the center is performed on the center of the polygon. If multiple polygons are selected, the center of the bounding box for the group is centered. The centering occurs as described below for each command. The Center commands are only enabled if objects in the circuit are presently selected.
For Object - Center - Vertically, the selected polygon(s) are moved to the center of the substrate in the vertical plane, but are not moved horizontally as shown in the illustration above.
For Object - Center - Horizontally, the selected polygon(s) are moved to the center of the substrate in the horizontal plane, but are not moved vertically as shown in the illustration above.
For Object - Center - Both, the selected polygon(s) are moved to the center of the substrate both planes as shown in the illustration above.
Pictured below is a group of polygons which were centered horizontally. Note that since multiple polygons were selected, the center of the bounding box of the group was centered. In the original position shown on the left, the bounding box is drawn in red.