The RGS and EPIC instruments usually work together giving observers the opportunity to perform joint analysis of strictly simultaneous high-resolution RGS data and lower-resolution EPIC data of higher statistical weight. The potential advantages of this type of work are considerable. Comparison of the independently derived calibration of the X-ray instruments  shows agreement to within a few percent where spectra overlap, as summarised at the 2010 XMM Users Group . While this level of agreement is highly competitive, it is greater than statistical errors. A technique, known as RGS-pn rectification, has been developed for observers who would like to reconcile RGS and EPIC-pn data even better than the few percent differences that remain from independent calibration. The remaining differences probably contain contributions from one or more of the following undoubtedly incomplete list of possible causes: RGS calibration inaccuracies; EPIC-pn calibration inaccuracies; data analysis procedures; spectral model inadequacies. The net result  of imposing agreement between RGS and EPIC-pn spectral models of a sample of AGN spectra assumed of simple form has been tabulated in a CCF component to be applied on explicit request as an extra correction to the RGS response matrix only. To reiterate for the sake of clarity, there remain small energy-dependent calibration differences that are manifested in statistical inconsistencies between models of simultaneous RGS and EPIC-pn data. These are likely to have resulted from a combination of factors (that are under continuing investigation). However, setting the switch,
rgsproc .... withrectification=true ....will implement specific cross-calibration information to apply a further adjustment to what would otherwise have been the best RGS calibration available, with the unique and specific aim of improving the reconciliation of simultaneous models of RGS and EPIC-pn data.