Information content of low-resolution spectra for a B star. Each panel shows the gradient spectrum ∇_l : = ∂f_λ/∂l with respect to different labels, l, derived from finite differencing of Kurucz model spectra (black) at T_eff = 18 000 K, log g = 4.0, [Fe/H] = 0, and vsin i = 100 km s⁻¹. The gradient spectra generated from HOTPAYNE are overplotted in red, illustrating the robustness of HOTPAYNE: the good agreement to the Kurucz model suggests that HOTPAYNE determines the labels from the relevant features instead of harnessing the astrophysical correlations. For T_eff, v_mic, vsin i, and N_He/N_tot, the gradient spectra are multiplied by 1000 K in T_eff, 5 km s⁻¹ in v_mic, 20 km s⁻¹ in vsin i, and 0.05 in N_He/N_tot for better visibility. This figure shows that the diagnostic signatures of label changes are widely distributed across the optical spectral range. Nonetheless, most spectral features are subtle due to LAMOST’s spectral resolution and the high effective temperature of the star: most abundance signatures are at the 1% level, even for abundance changes of 1 dex. Useful information about hot stars can only be extracted from a global fit to the spectrum and a careful spectral modeling.