Two diagnostic diagrams ([N II]-BPT, Baldwin et al. 1981; [S II]-BPT, Veilleux & Osterbrock 1987) illustrating the relative positions of the emission-line region models and the observed nebulae in M33 across three different line ratio spaces. Green stars mark the identified M33 H II regions, blue squares the M33 PNe, and red circles the M33 SNRs. Similarly, green contours show the H II regions models, blue ones the PN models, pink-red ones the SNR models, and yellow-orange ones the DIG models. On the left, the [N II]-BPT diagram (Baldwin et al. 1981) includes the Kewley et al. (2001) and Kauffmann et al. (2003) (dashed line) diagnostic lines. On the right, the [S II]-BPT diagram includes the Kewley et al. (2006) diagnostic line.