UV-diagnostic diagram showing the EW of C III] versus the ratio C III]/He II. The SFG and AGN models are from Nakajima & Maiolino (2022), while the dashed black line is from Nakajima et al. (2018). Values measured from the stacked spectra are shown as purple (weak stack), light green (strong stack), and pink (no C III] stack) points, with lower-redshift stacks (darker green) from Llerena et al. (2022). The C III] emitters with a broad Hα are plotted as red diamonds. Four galaxies that lie outside the range of the plot and have upper limits on He II emission are GDS-5093, RUBIES-49140, CAPERS-101195, and CAPERS-9244, with C III]/He II values of 0.04, 0.09, 0.02, and 0.01, respectively; the EWs of C III] are listed in Table C.1.