Objects can be also relatively positioned horizontally. This means that the objects can be horizontally re-located to a position that is a settable proportional distance across the enclosing frame width. A positioning around 50% would center the object in the enclosing frame.
The origin for the horizontal positioning is the left hand side of the frame which would enclose all selected objects.
After clicking the function icon, a dialogue box appears in which you can enter the relative position in percent.