plotpointfile is defined in your parameter file, potfit will write this file after the optimization is complete.
The plotpointfile contains a special representation of the potentials, only including the datapoints corresponding to parameters in the optimization. If you need the interpolated potentials, take a look at the plotfile.
The format is very simple, the potentials are written in the same order as in the potential file and are separated by two empty lines.
# distance potential_value internal_parameter_number .800000e+00 1.371272e+00 2 2.005203e+00 7.580872e-01 3 2.210407e+00 3.380089e-01 4 2.415610e+00 6.320161e-02 5 ...
Here is an example, showing a potential that was optimized at 25 sampling points.
This feature is only available for tabulated potentials.