Relative velocity of the main ISM absorption lines with respect to CIII]-λ1908 Å taken as systemic velocity for the SFG population and samples with increasing EW(CIII]). CIII] is plotted in black, and depending on the sample, SiII-λ1260 is drawn in cyan, SiII-λ1303 in green, CII-λ1334 in yellow, 1526 in orange, and 1670 in red. The mean outflow velocities (see text) are indicated as the long-dashed magenta lines.