What is symmetry?

Geometry Elements- Box Settings

If a circuit is symmetric about the center line parallel to the X axis, then activating symmetry results in 4X - 8X faster analysis and reduces the memory required to 1/4 to 1/3 of what was needed before symmetry was turned on.

When symmetry is turned on, everything below the line of symmetry is ignored, and all metal above the line of symmetry is "reflected" about the symmetry line. Therefore, you may enter in just half of your circuit, making sure that you enter in the half that is above the line of symmetry.  

Special care should be given to ports when using symmetry. Please refer to the "Ports" chapter in the User's Guide for a detailed explanation of port placement when symmetry is used.

You turn on symmetry in the Box Settings dialog box, opened when you select Circuit => Box.