EWsof [CIII] (left) and CIV (right) as a function of metallicity comparing observations (black dotted points) with our model predictions (colored curves) for star-forming galaxies. The observations consist of nearby and high-z galaxies compiled from the literature as shown in the legend of the left panel. The models use the single- (solid) and binary- (dashed) stellar populations at the starburst age of 1 Myr adopting several ionization parameters with a different color, as shown in the top-left panel of Fig. 11. Four objects from Amorín et al. (2017) that will be discussed in Sect. 6.2 with IDs: 510583858, 5101421970,5100998761, and 5100565880 are labeled as (a), (b), (c), and (d), respectively.