rmfgen generates the o/p rmf in the format specified by OGIP. This format allows response data for a given energy range to be stored in the form of contiguous groups of elements whose values are above a particular threshold. The values below threshold are not written. The fitting package expands this format, replacing those elements in between groups with zero. This is extremely useful in the case of a very sparse response matrix, where values close to zero are of no interest.
The threshold value can be set by the user through the threshold parameter and allowed range is between 0 and 1 (the maximum possible response value).