For a regression y|\textbf{x}, the structural dimension, d, is the smallest number of distinct linear combinations of \textbf{x} necessary to preserve the conditional distribution of y|\textbf{x}.
If \mathcal{S} has basis \eta and is a minimum DRS, then a plot of y versus \eta^T\textbf{x} is a minimal sufficient summary plot, and it is (d + 1)-dimensional.