Fraction of the total haloes in a simulation formed by 20 to 500 particles, as a function of the particle resolution, M_part, of a simulation. This fraction has been calculated using the Tinker et al. (2008) halo mass function with Planck-18 cosmological parameters (Planck Collaboration VI 2020) and varies for different redshifts (see labels in the figure). The vertical dashed lines of different colours indicate the location of state-of-the-art simulations, as indicated in the legend. The vertical solid green corresponds to UNIT, the simulation used in this work (Sect. 2). Over 90% of the haloes in these simulations have unresolved halo properties and this number increases for higher redshifts.