The Hydraulic Lookup Table form for Rating Curves the rating curve boundary condition have has similar functions with to the general form discussed in External Forcing Data . There are options to view all layers or to view each layer individually.
The US Only option means that flow through the structure depends on the upstream stage only (free flow) while US and DS means that flow through the structure depends on both upstream and downstream stages (submerged flow).
Figure 1. Boundary Data Series form for Rating Curves.