Number of clusters (inside the catalogue’s completeness limits around the Sun) as a function of the observed cluster’s Roche-volume filling factor − λ. The distribution of the observational OC data from Gaia DR3 is shown in light red, and the MWSC catalogue is shown in light blue. The different panels show the theoretical distributions for different SFE models (with r_apo = 10 kpc). The lines with different colours show models with different orbital eccentricities. The dotted lines present the results for the non-planar orbits (Z₀ = 200 pc). The cyan lines show the results for the three special models with the half-reduced initial λ₀ = 0.045.