The relative alignment, i.e., the offsets with respect to each other of the different XMM focal instruments, determines where a source that is ``on axis'' for one particular X-ray telescope will be imaged in the others. For most applications, off-axis angles of order 1' are not important, in particular for EPIC full window mode observations. However, RGS has a much smaller ``field of view'' or rather area for optimal placement of the dispersed spectra on the elongated CCD array. For best energy and flux calibration, source spectra should be optimally centred. Therefore, if high-resolution spectroscopy (i.e., observations with an emphasis on RGS science data) are required, RGS should be specified as the primary instrument. The same holds for EPIC and OM, if operated with small science windows. In case EPIC is prime, it is recommended to choose the pn camera as the prime instrument to avoid the inter-CCD gap at the centre of the pn array.