The BM@N is a fixed target experiment aimed to study (multi) strange hyperons production and to search for hypernuclei in nucleus-nucleus collisions at beam ion energies up to 4.5 A GeV. The BM@N is also the most suitable experiment for the study of fragmentation in heavy ion collisions. The BM@N powerful analyzing dipole magnet deflects the charged spectators and partially separates them from the neutron spectators. The separate measurements of charged and neutron spectators allow to determine the geometry of nuclear collisions (centrality and reaction plane orientation) as well as to study the mechanisms of fragmentation and the equation of state of nuclear matter. The measurements of the heavy charged fragments and proton spectators will be performed with the quartz and scintillator hodoscopes correspondingly, while the forward hadron calorimeter will be used to measure the neutron spectators. Results of simulation of the proposed forward detector system are discussed, showing that the new experimental fragmentation data for heavy ion collisions are needed to validate fragmentation models.