The Periodic boundary condition defines a special type of symmetry that obeys cyclic/repeating condition of the flow across the boundary surface. For this condition it is mandatory to select two boundary faces that will be treated as if they are physically connected. The flow exiting/entering from one face then enters/exits the other face.
Some examples where this could be useful are within heat exchanger tube bundles or a swept vane pump impeller.
Important
The faces do not need to be in the same orientation, but the two periodic boundary condition faces should have identical size areas.