To Mirror References by Using a Fence

Place a fence around the area containing the references to be mirrored.
From the Model tab of the Project Explorer dialog, select a reference.
In the Reference toolbox, click the Mirror Reference tool.
In the tool settings window, choose the Method: About Vertical Line or About Horizontal Line.
Turn on Use Fence and choose the fence mode.
Enter a data point about which the references are to be mirrored.