Accreted stellar-mass fraction in 3D radial bins, f_acc(r) = M_acc(r)/M_*(r), for the illustrative example galaxy TNG50-714463. The vertical green dashed line indicates the stellar half-mass radius of the galaxy. Accreted stars begin to dominate the stellar mass (f_acc > 0.5) at r ≳ 15.5 kpc. This defines the ‘crossing radius’, r_cross, of the galaxy. This radius is more than seven times larger than the stellar half-mass radius of this galaxy and is smaller than our adopted definition of ‘galaxy radius’, r_glx, shown by a dashed blue line. Changes of r_glx by a factor of two (blue shaded regions) produce only small variations in the accreted fraction profile (grey shaded regions), with r_cross = 15 ± 1 kpc.