Fitting packages such as XSPEC do not allow the channel range of the spectrum and that defined in the RMF to differ. XSPEC assumes the spectral range to correspond to the legal range of the CHANNEL column, ie the attributes TLMIN1 and TLMAX1.
If the channel range of the spectrum and that of the reponse data obtained from the CAL are found to differ, rmfgen raises a warning and performs one of the following corrective actions:
Note that the only accepted value for TLMIN is zero (which is the same as the lower bound for the CAL data, ensuring that the spectral channel range can only be a subset or superset of the response channel range, and not overlapping).
XMM-Newton SOC -- 2021-11-30