After fitting some function to some data one can retrieve the fit status via, e.g.
r = hSpectrumCalibration->Fit(f1, "R0MS");
status = r->Status();
But what do those values of
status actually mean?
So far I belive 0 is CONVERGED, negative values are FAILED and 4000 is OK.
But I could not find any information in the documentation  on that, beside other people asking for the same thing a year ago .
 Fit status documentation
I think it depends on the minimizer algorithm used, that’s why the documentation doesn’t explicitly say what the different values mean.
You can use IsValid() to get a boolean yes/no, otherwise you’ll have to dig into the specific minimizer algorithm.
Edit: I just tried to look for that information for TMinuit and the Linear fitter, but I didn’t get very far.