Completeness (color scale) as a function of the flux peak and the FWHM of a line. The four diagrams correspond to the R_{< 30}, R₃₀₋₅₀, R₅₀₋₇₀, and R₇₀₋₉₀ regions respectively. As is evident from their comparison, the completeness is a strong function of the line location in the FOV because of the degrading sensitivity from the phase center to larger radii. The lines used to build the [C II] LF (see Sect. 5) are also shown (filled circles), except for the two brightes ones (i.e., S848185 and S842313), which are located outside the plotted ranges and have completeness equal to 1 everywhere in the FOV. We show the [C II] serendipitous detections in all the panels, independently from the line location in the FOV, since we computed their completeness in each ring when building the LF (see Eq. (3)).