Upper panels: histograms of median relative flux error (σ_med) for the spectra in the Clean samples (left) and the Uncertain samples (right). Dashed lines and purple shaded areas indicate the median values and ±1 standard deviations of the distributions. Lower panel: distribution of relative flux error at each spectral bin (σ_λ = ΔFlux(λ)/Flux(λ)) for the Clean (red) and Uncertain (blue) groups. The shaded areas indicate the interquantile ranges from 16% to 84%. The red and blue dashed lines show the median of σ_med for each group as shown in the upper panels and the black dashed line shows the median σ_med for all 2051 spectra. The green dotted line indicates the upper boundary of the training range (log σ = -0.5) of our cINN. The narrow grey-shaded vertical areas indicate the masked spectral bins not used in our network due to the presence of emission lines.