Illustration of the four-population model. The left side displays the situation before the collision: two pre-planetesimals collide with impact velocity v₀ and impact parameter b. The target and projectile are characterised by their defining quantities, e.g., mass m, filling factor φ, and rotational energy E_rot. Depending on these parameters, an outcome population is generated in the collision (middle). The four-population model distinguishes the fragments according to their mass. The population classes are: a) the largest fragment; b) the second largest fragment; c) the power-law population; and d) the sub-resolution population. This categorisation is sufficient to describe the outcome types of Fig. 1 as well as intermediate outcomes.