By default an ARF is generated at a set of energies specified within the CCF and is compatible with the default energies produced by rmfgen. To use different energies, contained within an rmf file, m1_r5_all_15.rmf, use the command:
arfgen spectrumset=spectrum.pha detmaptype=dataset detmaparray='detmap.ds'
withrmfset=true rmfset=m1_r5_all_15.rmf