LLE photon index as a function of the spectral lag between Band 1 (GBM-NaI) and Band 4 (LAT-LLE) for 56 out of 70 GRBs in our sample. Short GRBs are indicated by stars. Green points represent GRBs that deviate from the expected trend, that is with τ₁₄ < 0 and LLE photon index <  − 2 (considering the uncertainties on their values), corresponding to GRBs 100724B and 170405A. The dashed-blue line represent the arbitrary threshold between hard and soft spectra in the framework of the phenomenological scenario, described in Section 7. Spectral lags τ are reported in symmetric-logarithmic scale for visualization purpose.