Overview of He and metal classification diagnostics for Galactic OB stars. Symbols are circles for luminosity classes V–IV, squares for III–II and triangles for I, filled for primary diagnostics, open for secondary diagnostics. Upper panel: Overview of He I–II and Si III–IV classification diagnostics for O and early B templates (Sota et al. 2011; Negueruela et al. 2024). Lower panel: As above, for Si II–IV and He I/Mg II classification diagnostics for B stars (restricted to supergiants for B2.5+), excluding B4, B6 subtypes for LMC stars (Fitzpatrick 1991). We also show the ratio of Si III λ4553 to Mg II λ4481 for B2–3 supergiants (green) since this is the qualitative diagnostic for SMC mid-B supergiants with B4, B6–7 excluded (Lennon 1997).