Answer:
In RF‑CONCRETE Surfaces, the "Top" or "Bottom" position is controlled by the orientation of the local coordinate system of the surface.
See also FAQ 000740:
For a wall, this means that As,bottom or As,top can be the reinforcement on the outside or inside depending on the orientation of the local z-axis.
Both values must be considered for the reinforcement design.
That is, the sum of As,bottom + As,top.