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The distribution of galaxies in optical diagnostic diagrams can provideinformation about their physical parameters when compared with ionizationmodels under proper assumptions. By using a sample of central emitting regionsfrom the MaNGA survey, we find evidence of the existence of upper boundariesfor narrow-line regions (NLRs) of active galactic nuclei (AGN) in optical BPTdiagrams, especially in the diagrams involving [S II]$\lambda \lambda$6716,6731/H$\alpha$. Photoionization models can well reproduce the boundaries as aconsequence of the decrease of [S II]$\lambda \lambda$6716, 6731/H$\alpha$ and[O III]$\lambda$5007/H$\beta$ ratios at very high metallicity. Whilst the exactlocation of the upper boundary in the [S II] BPT diagram only weakly depends onthe electron density of the ionized cloud and the secondary nitrogenprescription, its dependence on the shapes of the input SEDs is much stronger.This allows us to constrain the power-law index of the AGN SED between 1 Rydand $\sim100$ Ryd to be less than or equal to $-1.40\pm 0.05$. The coverage ofthe photoionization models in the [N II] BPT diagram has a stronger dependenceon the electron density and the secondary nitrogen prescription. With thedensity constrained by the [S II] doublet ratio and the input SED constrainedby the [S II] BPT diagram, we find that the extent of the data in the [N II]BPT diagram favors those prescriptions with high N/O ratios. Althoughshock-ionized cloud can produce similar line ratios as those byphotoionization, the resulting shapes of the upper boundaries, if exist, wouldlikely be different from those of a photoionizing origin.

Upper boundaries of AGN regions in optical diagnostic diagrams