Curtain Wall Members

The Reference Line is the original input line/polyline/arc you draw in the window. If you use a Boundary method to create a Curtain Wall, the first drawn segment of the Boundary is the Reference Line.
The Base Surface defines the shape and orientation of the Curtain Wall. It is the surface in which the physical members of the Curtain Wall (Frame, Panels) lie. The Base Surface may coincide with the Reference Surface; more likely, you will offset it. When you move or rotate the Curtain Wall as a whole, it is the Base Surface which you are moving; the Reference Line and Reference Surface move along with the Base Surface.
The Base Surface is crisscrossed by a Grid (primary gridlines plus secondary gridlines, defined as part of the Curtain Wall Scheme.)