Rules for in‐medium complex particle production in nuclear reactions are proposed. These rules have been implemented in two models to simulate nucleon‐nucleus and nucleus‐nucleus reactions around the Fermi energy. Our work emphasizes the effect of randomness in cluster formation, the importance of the nucleonic Fermi motion as well as the role of conservation laws. The concepts of total available phase‐space and explored phase‐space under constraint imposed by the reaction are clarified. The compatibility of experimental observations with a random clusterization is illustrated in a schematic scenario of a proton‐nucleus collision. The role of randomness under constraint is also illustrated in the nucleus‐nucleus case.