In the Boundary Pair node, you supply information about the corresponding periodic boundaries. In 2D, you need at least two such pairs, and in 3D at least three pairs. The number of pairs depends on the shape of the unit cell. This node has no other settings that the boundary selection.

Select all corresponding boundaries belonging to both sides of the pair. If you need to manually specify the source and destination sides of the pairs, add a Destination Selection subnode.

Physics tab with Cell Periodicity node selected in the model tree:
Attributes>Boundary Pair