This very simple example demonstrates the effect of applying periodic boundary condition, which forces the field potential to be the same on opposite sides of the model.
A nonlinear plane-parallel problem of magnetostatics.
Two regions, A and B, have the same shape, volume loading and are surrounded by Dirichlet boundary condition, which does not allow the field to penetrate outside. Region B is also subdivided into B1 and B2, and the periodic boundary condition is specified on two sides, which makes these regions the continuation of each other. As a result, field distribution in both A and B must be equivalent.
This example also demonstrates that the mesh on the periodic boundary is not necessarily the same - please notice that the mesh spacing settings in four corners of the model are all different!
See the Perio1.pbm problem in the Examples folder. Perio1odd.pbm is almost the same, but for one difference: odd periodic condition is applied, which forces the field potential to be opposite on two sides of the region.