If you are fitting a tabulated potential, this file is used to declare which parameters are being fit.
Fitting the Tersoff potential, for example, yields the following REFPOT:
# Example of a Tersoff potential file for fitting # m, Inmma, lambda3, c, d, costheta0, n, beta, lambda2, B, R, D, lambda1, A Ge Ge Ge 3 1         2.95 0.15  
where the “” brackets designate parameters that are to be fit.
It is of use to define symmetries between parameters at times. If you want all of a certain parameter to be the same, use  for that set of parameters. Use  for the next set of symmetric parameters, etc..
An example is shown here:
Ge Ge Ge 3 1         2.95 0.15   Ge Ge Si 3 1   [-3] [-4]     2.95 0.15  
All the parameters with a number in the “” brackets will be treated as a single parameter during fitting.
The “” brackets with negative numbers will adopt the same magnitude of the parameters in  and , but with opposite signs. This is useful when fitting charges.