Symmetry boundary conditions can be used whenever the EM fields have a plane of symmetry through the middle of the simulation region. By taking advantage of this symmetry, the simulation volume and time can be reduced by factors of 2, 4 or 8.
This topic describes the difference between symmetric and antisymmetric boundary conditions, and how to select the appropriate boundary for your simulation.

How to: Take advantage of symmetry (YouTube)
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When the EM fields have a plane of symmetry, some field components must be zero at the plane of symmetry. Symmetry boundary conditions are implemented by forcing the appropriate field components to zero. The following table lists the field components that are zero for each symmetry option.

Symmetric Boundary 
AntiSymmetric Boundary 
Normal Electric Field 
ZERO 

Tangential Electric Field 

ZERO 
Normal Magnetic Field 

ZERO 
Tangential Magnetic Field 
ZERO 

The nonzero field components are shown in the following figure. Note that the blue arrows are electric field and the green arrows are magnetic field.
The electric and magnetic fields will obey certain symmetry rules with respect to reflections through the plane of symmetry. The reflection symmetry rules are shown in the figure below.
The above symmetry rules are helpful when determining the symmetry of modes found with the mode source, and resonant modes of cavities. The following figure shows the Real part of Ex and Ey of a mode that was calculated with the Mode source. The direction of propagation is +Z. Notice that Ex has the same sign on each half of the simulation region. Ey has the opposite sign on each half. Ey also goes through zero along the plane of symmetry.
Using the above symmetry rules, we can determine that there is a plane of antisymmetry at X=0, and a plane of symmetry at Y=0. Therefore, the x min boundary should be set to antisymmetric and the ymin boundary condition should be set to symmetric.
If the EM fields through a periodic structure have an plane of symmetry or antisymmetry in the middle of a period of the structure, then set the boundary conditions as follows:
1) select the option allow symmetry on all boundary conditions
2) set the minimum and maximum boundary conditions to be either symmetric or antisymmetric depending on the symmetry rules given above (in practice the boundary conditions are usually either both symmetric or both antisymmetric).
Referring to the example below, changing from Setting A to Setting B will preserve periodicity while reducing the computation time needed by about 4x. Again this only applies if the structure and fields are BOTH symmetric and periodic.
This section applies for FDTD and MODE' propagator, where sources are used to inject light into the simulation region.
Most sources show their Electric or Magnetic polarization with colored arrows.
•Electric field polarization is Blue •Magnetic field polarization is Green
In the image to the right, the electric dipole is on the left and the magnetic dipole is on the right. 
Symmetry boundary conditions use a similar color scheme.
•Symmetric BC are Blue
•AntiSymmetric are Green.
The following figure shows how to select the correct symmetry condition based on the source polarization.
•If the source polarization is tangential to the plane of symmetry, select the symmetry option with the same color.
•If the source polarization is normal to the plane of symmetry, select the symmetry option with the opposite color.
In the following example with the triangular structure, there is one plane of symmetry in X. The source polarization (Blue) is tangential to the X boundary. Therefore, we use Symmetric for the X min boundary condition. This simulation will run 2x faster than the equivalent simulation without any symmetry settings. Notice that the blue arrow of the source is along the edge of the region with the same color.
This next example shows a simulation with two planes of symmetry (in X and Y) The source polarization (Blue) is normal to the X boundary, and tangential to the Y boundary. Therefore, use AntiSymmetric for the X min boundary and Symmetric for the Y min boundary. The X and Y max boundary condition do not need to be modified. This simulation will run 4x faster than the equivalent simulation without symmetry boundary conditions. Notice that the blue arrow of the source is along the edge of the region with the same color, and sticking into the region with opposite color following the rules outlined above.
Warning: Using the wrong symmetry option When using symmetry boundary conditions, it is easy to select the wrong symmetry option. There will be no warning message if the wrong symmetry option is selected, but the simulation results will be completely wrong. The easiest way to confirm that you have the correct option is by rerunning the simulation without symmetry boundary conditions. Both simulations should give the same results. If they don't, you may have selected the wrong symmetry option. 
Note: Simulation span Do not change the size of the simulation region when using symmetry. If symmetry boundary conditions are selected, half of the simulation region will automatically be shaded in blue or green, indicating the portion of the simulation region that will not be directly simulated. 
Note: Unfolding data Script commands like getdata, getelectric or getmagnetic automatically unfold the field data according to the symmetry rules of the boundary condition. This makes it easy to create an image monitor the data over the entire simulation region, even though only half the simulation region was actually simulated. However, there will be no data recorded by the monitor if it is entirely in the shaded region. Some analysis group may not work when symmetry is applied but some of them contain script commands to properly unfold the field data. 