Parameter estimation is a critical task in the development of force-field models, and the quality of the parameters used in the force-field are integral to model performance. Within the formulation of HAIEFF models, atom types are assigned to each site in a molecule. A pair potential is used to describe the interaction between each possible atom type-atom type pairing. By keeping a consistent definition of the atom types, the parameters describing each pairwise interaction can remain unaltered between different molecules, hence the 'transferable' attribute of the repulsion/dispersion term in HAIEFF forcefields. Transferable potentials help provide practical limits on the number of parameters describing the force-field, although they can often lack an appropriate consideration of the effects of the local molecular environment on the repulsive/dispersive interactions. Within this general framework, the FIT and Williams parameterizations emerged as early forebearers of HAIEFF force-fields.