XMM-Newton has three independent X-ray telescopes and an optical/UV telescope, with a total of six science instruments. Neither the telescopes' optical axes nor the centre positions of the detector FOVs are aligned perfectly. Thus, radiation from sources in the spacecraft's boresight direction does not fall onto the nominal on-axis position of all detectors simultaneously.
Starting in AO18, all the observations are performed placing the target at the same position (that corresponding to 'EPIC-pn Prime'), regardless of the instrument selected as 'Prime' in the proposal. This choice does not represent any drawback from the point of view of the RGS spectral coverage, and ensures that the source is located far enough from the edge of the EPIC-pn chip.
The user can always choose an off-axis pointing, but this requires specification of a position angle which implies reduced visibility. The user can use the SAS task `srcinfov' (which is part of the `bstools' task package) to determine these choices.