2D construction lines used inside Revit's sketch modes to align, constrain, or set up geometry - distinct from a Work Plane, which is the 3D surface a sketch is drawn on. Reference planes are the invisible-in-3D guide lines a modeler adds inside a family editor or sketch (often locked with dimensions or parameters) to keep geometry aligned as it flexes - they're a sketch-time tool, while a Work Plane is the actual 3D surface hosting the whole sketch.