The program tabnllsqfit can fit (linear or non-linear, depending on the parameters) a function to a set of datapoints from an ASCII table. The keyword fit= describes the model (e.g. a line, plane, gaussian, circle, etc.), of which a few common ones have been pre-compiled with the program. In that sense this is different from the previous two function descriptors, which always get loaded from a directory with precompiled object files. The keyword load= can be used to feed a user defined function to this program. The manual page has a lot more details.