GRBs classification in 4 groups according to their distance to the E_pi – E_iso boundary. The parameter used to separate the 4 groups is their vertical distance (for GRBs with a redshift) or their smallest vertical distance (for GRBs without a redshift) to the black line. The black line has a slope = 0.486, assumed to be the slope of the E_pi – E_iso boundary. The position of the black line is arbitrary, it does not impact the groups as long as it stays below the GRB sample.