EW(CaT) vs. BP − RP color-coded by density for all selected RVS objects. Overlaid black dots are those with APOGEE measurements of log g. They are plotted separately to represent the positions of different types of stars. The blue dots in the left panel are the known Galactic RSGs mentioned in Sect. 3.1.1. The red asterisks mark the positions of zero-extinction K1 and M2-M5 RSGs. The solid red lines represent the bluest boundary of Galactic RSGs in this diagram, while the dashed red line marks the dividing line between the early- and late-type RSGs.