Mean spline to quadratic function ratio ⟨ R ⟩ vs. the coefficient a of the quadratic function for all UKIDSS sources satisfying the SED matching criteria of Eq. (2) and with a > 0. The sources are color-coded according to their visual evaluation of good (pale sky blue) or bad (red) match with the MIR SED. The dashed lines indicate the limits on ⟨ R ⟩ and a defining the conditions of Eq. (B.1) to distinguish the unambiguous sources from the ambiguous sources.