Diagram illustrating the trend of Z across different spectral types, with the reference values for each spectral type obtained as the geometric mean of all the measurements assumed for all the objects belonging to a specific spectral type, including our method, the use of the CIV/Hβ ratio, and the presence of microturbulence for all sources belonging to the A2, A3, and A4 spectral types. Confidence intervals are obtained as the standard deviation of our results. The number of objects contributing to each spectral type is shown in green, along with the average R_FeII of the sources considered. Two reference points (in red) are shown describing the RL and RQ Pop. B sources studied in M23.