Physical processes setting the boundaries for BH masses and spins for the CHE formation scenario. The four colored areas indicate excluded regions due to four different physical processes. The M_TAMS and t_merge limits apply only to CHE, while the Γ − Ω and (P)PISNe boundaries apply in general to all field formation scenarios. In this figure an assumed limit for the M_TAMS of CHE stars is set at 30 M_⊙, which sets the boundary at the left of the figure. Precise boundaries depend on the metallicity used to determine stellar properties in the semi-analytical model used, which in this case were taken from calculations at Z_⊙/50.