The Reference Direction node defines the first reference direction vector a1, discussed in the Periodic Structure documentation.

In 3D, select an edge. The direction a1 will be aligned with the selected edge. In 2D, this selection is not available. Instead, the direction a1 will be aligned with the parent Periodic Structure Excited Port Selection,

This selection is only available in 3D. The direction a1 will point away from the selected point.

This selection is only available in 2D. The direction a1 will point away from the selected point.