Emission-mechanism classifications in the mass–freefall velocity plane. Red circles, blue crosses, green bars, and gray squares denote the four main classes defined in Section 3.3: S, S + BC, S + BC–like, and N, respectively. Objects with the auxiliary flags for large observational uncertainty (U) or BC dominance (DBC) are shown in black, and the unlikely inferred property (Q) is in transparent color, while retaining the symbol of their main class. Thin gray lines indicate isocontours of object radius: 0.2, 0.5, 1, and 2 R_⊙, from left to right. Note that points are plotted in the legend order from top to bottom, with the upper entries drawn in the foreground and shown with smaller symbols, so that they are not obscured by the larger symbols plotted behind them. Also, multiple observations of the same object are plotted separately.