  We present new features of the transition from nuclear multifragmentation to
neck fragmentation in semi-central heavy-ion collisions at Fermi energies as
obtained within a microscopic transport model, Stochastic Mean Field (SMF). We
show that along this transition specific hierarchy phenomena of some kinematic
observables associated with the intermediate mass fragments develop. Their
correlations with the dynamics of isospin degree of freedom, predicted by our
calculations, open new possibilities to learn about the density dependence of
nuclear symmetry energy below saturation, as well as about the fragmentation
mechanisms. Detailed results are presented for mass symmetric Sn + Sn reactions
with different isospin content at 50A MeV .
