Examples of sources with various redshifts and associated confidence levels. From top to bottom, we show spectra (in black) and associated 1σ noise (in gray) with (i) confidence 1, with tentatively [O II] at low S/N, (ii) confidence 2 with the [O II] doublet spectrally resolved with a correct S/N, (iii) confidence 3 with multiple emission lines at high S/N, (iv) confidence 3 with strong continuum features, and (v) confidence 3 with the [O II] doublet with a high S/N and Fe II and Mg II absorption lines. The main spectral features expected in either absorption or emission are indicated by red and blue vertical lines, respectively. Zoom-in of the normalized rest-frame spectrum around the [O II] doublet is shown to the right of each spectrum, as well as the MUSE [O II] narrow band image obtained from collapsing the continuum-free cube over ±5 Å at rest-frame, and the HST-ACS image in the F814W band. Displayed spectra have been extracted on 1″ diameter apertures and smoothed using a two-spectral-element FWHM Gaussian kernel. Images are 2″ wide.