For symmetry conditions, you can use them whenever both geometric and electrical symmetries are consistent. Geometric symmetries are easy to tell, but electrical symmetries might not be permitted in all conditions. For example, if you have 4 spheres located on the corners of a square, then there is an easy quarter geometric symmetry due to the box geometry, but the electrical symmetry would depend on the voltages applied to each sphere, and would only be true for certain combinations of equal and/or opposite voltages on the 4 objects.
Similarly for periodic structures, you could use symmetry boundaries when there is appropriate electrical symmetry conditions (fields either completely tangential or perpendicular to a plane). If the fields are not parallel/perpendicular to any periodic plane, then you can use master/slave BCs on planes where the geometry repeats. If the electric fields are completely between the two parallel plates (no fringing), then you only need to model the area between the plates with the top/bottom faces assigned as 2 different voltages, and the side faces of the vacuum assigned as symmetry/periodic. If you do have fringing on the sides, then you would have a larger region object, and you can assign the symmetry/periodicity directly the larger region face.
The U-V vectors for master/slave boundaries just need to be consistent from master to slave boundaries. They can be defined in arbitrary was on the first face, but need to be reproduced exactly on the second face. They would define if the symmetry is translational or if there is a rotation.
Once you define the voltages, then you can define the capacitance matrix definition (select all voltage assignments). Once you solve the model, make sure to scale the result by the symmetry factor, and (in 2D) also by the model depth (default 2D assumption is 1meter). The solutions are found in the Solution Data or in the Result Plots sections.