Spectral indices plotted as α_{1.5 − 5} vs. α_{0.144 − 1.5}, α_3 − 5 vs. α_{1.5 − 3}, α_5 − 10 vs. α_3 − 5, and α_5 − 10 vs. α_{1.5 − 5}. Markers are colored according to the dominating mechanism responsible for the detected radio emission (Table 2). The dashed line indicates the 1 : 1 ratio of equal slopes, while the horizontal and vertical dotted lines correspond to α = −0.5. The region showing flat or inverted spectra, which may be associated with a core-dominated jet or a corona, is shaded gray. The region with a spectral index α = −0.8 ± 0.4 and a scatter of 0.2, possibly related to SF, is shaded turquoise. The region showing very steep spectral slopes with α_5 − 10 < −2 in the right-hand panel, which may imply possible relic emission, is shaded blue.