You should gather all reaction products that are split between genstk.inc and fheavy.inc, as already explained. Also, you can cross-check that nothing was forgotten, out of the neutron induced reaction, by looking at the target nucleus’ mass and atomic numbers (IBTAR and ICHTAR in resnuc.inc).
Could you kindly clarify whether, for low-energy neutron reactions (icode = 3xx), it would be possible to further distinguish the specific reaction channels—such as elastic scattering, inelastic scattering, and neutron capture?
Yes. Note that for low-energy neutron interactions the only available ICODE value is 300.
Still, if you look at the reaction products as earlier mentioned, you can naturally figure out the specific reaction channel: if you find just one proton and possible gammas (in genstk.inc) and a residue (in fheavy.inc), it was (n,p); if you find just one neutron and some de-excitation gamma (in genstk.inc) and a residue matching the target nucleus (in fheavy.inc), it was (n,n’); if you find nothing in genstk.inc and a residue matching the target nucleus in fheavy.inc, it was (n,el); if you find just gammas (in genstk.inc) and a residue (in fheavy.inc), it was capture, namely (n,gamma); and so on for any possible channel.
Therefore, gathering all the interaction products, you have the exact picture of the reaction channel.