A Reference Plane can be set in the 3D History on base planes, reference planes and fixed planes via context menu command Insert new plane.... See Section 7.6.2.2.1, “Create reference planes via 3D History ”.

Furthermore reference planes can be created in the 3D view when selecting a face via context menu command Create plane on selected Face. See Section 7.6.2.2.5, “ Create reference plane via 3D view ”.