CSS stands for the Complexation Significance Score, which indicates how significant for assembly formation the interface is. The score is defined as a maximal fraction of the total free energy of binding that belongs to the interface in stable assemblies. For example, if the only stable assembly found is a tetramer with 2 interfaces #1 with ΔG=-10 kcal/M and 2 interfaces #2 with ΔG=-5 kcal/M, then CSS for interface #1 is 2×10/(2×10+2×5)=2/3. If, in addition, a dimer with interface #1 is found to be stable, then CSS for interface #1 is max(1,2/3)=1.

The service also provides data for assemblies with stability in a "grey" area of complexation criteria. In this area, CSS is decreased by factor of 10.