arfgen computes a vector of areas for a given set of energy ranges. These energy ranges are by default taken from the CAL.
Alternatively, the arfgen can take the energy ranges from the ENERG_LO and ENERG_HI columns of an RMF dataset. This feature is useful if, for example, the user wants to make sure his/her RMF and ARF datasets are in sync. This can be done by setting the parameter withrmfset to true and specifying a dataset name for the rmfset parameter.