Event fraction distribution as a function of the normalized charged-particle density in PbPb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Centrality classification is based on $N_{\rm ch}$ (red circles) and $\eta$ (blue squares) in $|\eta|\leq0.8$, and on forward $N_{\rm ch}$ for the VZERO (black crosses). $\langle {\rm d}N/{\rm d}\eta \rangle^{\rm norm}$ is derived from the extrapolated spectra in $|\eta|\leq0.8$. Gray curves represent fits using the model from Ref. . The positions of $\langle {\rm d}N/{\rm d}\eta \rangle^{\rm norm}_{knee}$, indicated by dashed vertical lines, are 1.137, 1.120, and 1.111 for the $N_{\rm ch}$, $\eta$, and VZERO-based centrality estimators, respectively.