Flowchart summarising the decision process adopted to compute nitrogen and oxygen abundances depending on the available spectral features. The first step is to assess whether the dataset includes detections of auroral lines. If two oxygen auroral lines are detected, the direct method is applied: both T₂ and T₃ temperatures and the electron density are derived, providing all the quantities needed to compute nitrogen abundances and metallicities. If only one auroral line is available, the corresponding temperature is measured directly, while the second is inferred from a T₂ − T₃ relation appropriately calibrated using a local-based relation for nearby galaxies, or the C25 calibration for high-redshift objects. If no auroral detections are present, the procedure diverges between local and high-redshift samples. For high-z galaxies, metallicities are obtained from strong-line calibrations of C25, and nitrogen abundances from the corresponding high-z N/O–strong-line relations. For local samples, metallicities are derived from the Curti et al. (2017) calibrations, while nitrogen abundances follow N/O–strong-line relations calibrated on SDSS local stacks. The local sample in this figure refers to the dataset described in Section 2.1.3, and JWST refers to the Sample depicted in Section 2.1.2.