Location in three BPT diagrams of photoionisation simulations (top row) and ionised nebulae from PHANGS-MUSE (C23, bottom row). In each panel, the colour-coding represents class labels (green: H II regions, blue: PNe, and red: SNRs). The class labels for the observed nebulae are based on the Bayesian methodology of C23 and are never used in the training, as we treat the target domain as unlabelled. Only the subset of the C23 catalogue used to train the classification algorithm presented in this work is shown. In each BPT diagram we show the demarcation lines of Law et al. (2021) (L21, solid line) and Kewley et al. (2001, 2006) (K01, dashed line). The Kauffmann et al. (2003) line in the [N II]-BPT is almost coincident with the Law et al. (2021) line. The figure demonstrates that there is only a limited overlap in parameter space between simulations and data.