Relations between stellar haloes masses and masses of accreted systems. Top: mass of the most massive merger (M_mmm) relative to the accreted mass vs mass of the most massive merger. Bottom: In situ halo mass fraction versus the total accreted stellar mass. Different symbols show different M31/MW HESTIA galaxies. Shown are the kinematically defined components using Toomre diagrams (in blue), while the total mass without any kinematics selections (in red). Depending on the definition, the single most massive merger contributes from 20% to 70% of the total accreted mass (see also, Cooper et al. 2010; Monachesi et al. 2019). At the same, the in situ mass fraction correlates well with the total accreted mass.