Optical O III/Hβ versus N II/Hα (top) and S II/Hα (bottom) diagnostic diagrams from Baldwin et al. (1981) and Veilleux & Osterbrock (1987), respectively. Continuous and dashed curves show the demarcation criteria by Kewley et al. (2001, 2006) and Kauffmann et al. (2003), respectively. The dotted line indicates the traditional S II/Hα cutoff of 0.4 for SNR (e.g. Long et al. 2018). Green stars are the integrated measurements of H II regions, blue circles of PNe and red squares of SNRs as labelled in the legend. The H II regions with yellow edges host WR stars. Black dots indicate the position in the diagram of three H II regions (IDs 112, 113 and 120) which are discussed in more details in Sect. 4.3.