THERM uses one structural dependent correction (Hydrogen Bond Increment - HBI), which include vibrational moments, disassociation energies, and rotational moments, based on the structure around the radical.
THERM and THERGAS are similar (at least in this implementation) in that both use Benson rules for the parent molecule, i.e. where hydrogen is added to the radical. In addition, the corrections, due to the loss of the hydrogen, are basically the same for both methods. The main difference is that THERM collects all the corrections in one term, the hydrogen bond increment (HBI) and THERGAS calculates these corrections explicitly. In the formation of the HBI correction, there are some differences in how and what corrections are made.
There are several updates to the original set of HBI corrections since the original paper of Bozzelli.