Importance (r²) of the observational parameters in predicting the ex situ stellar mass fraction f_exsitu. The top panels are models trained and validated by TNG100 and EAGLE, respectively; the bottom panels are models cross-validated with TNG100 and EAGLE. In each panel the two black points shows the importance of combined parameters: Sub0 including ∇ρ_outer, ∇(g − r)_outer, f_outerhalo, f_innerhalo, M_r, r₉₀; Sub1 including the same, but not f_innerhalo. The rest of the points show the importance of each single parameter as labelled. The error bars in the top two panels are the scatter of results from different training and validating sets and different hyperparameters of the random forest models; the error bars in the bottom two panels are from different hyperparameters of the random forest models.