Symmetry boundaries are used to reduce the extent of your computational model to a symmetric subsection of the overall physical system. Figures 7.14.1 and 7.14.2 illustrate two examples of symmetry boundary conditions used in this way.
Figure 7.14.3 illustrates two problems in which a symmetry plane would be inappropriate. In both examples, the problem geometry is symmetric but the flow itself does not obey the symmetry boundary conditions. In the first example, buoyancy creates an asymmetric flow. In the second, swirl in the flow creates a flow normal to the would-be symmetry plane. Note that this second example should be handled using rotationally periodic boundaries (as illustrated in Figure 7.15.1).