Hierarchy of model categories examined by RTModel. 1L1S means single-lens-single-source, 1L2S is single-lens-binary-source, 2L1S is binary-lens-single-source. “+p” indicates the presence of parallax, “+x” is xallarap, and “+o” is lens orbital motion. The arrows go from lower-dimensionality models to higher-dimensionality models that include them as special cases (nested models).