In particular the RGS, but also the OM with its grisms, can place position angle constraints on observations, if one wants to avoid spatial overlaps of the dispersed spectra of adjacent sources. Therefore, adjacent sources should not be aligned with the dispersion direction. In the case of RGS, avoidance of bright sources outside the field of view, as described in § 22.214.171.124, can also lead to position angle constraints. A description of how to avoid such sources is provided in § 4.4).
If nothing is specified during proposal submission, there is by default no orientation constraint on XMM observations. Note that any constraint imposed on an observation make it ``time-critical'' and thereby more difficult to schedule.