Lead scandium niobate (PSN) belongs to the class of perovskite-type relaxor ferroelectrics, which are among the key functional materials. Due to their structural complexity, the atomistic mechanism of formation of relaxor state is still unclear. The aim of the proposed high-pressure neutron diffraction study is three-fold: (i) to reveal if pressure-induced octahedral tilting, observed by us in two other compounds, is a common relaxor feature; (ii) to check whether the existence of chemically B-site long-range ordered regions, deduced from high-pressure XRD and Raman scattering, is indeed obscured by incoherent polar displacements at ambient conditions; (iii) to reveal what triggers the pressure-induced out-of-phase octahedral tilting in these highly inhomogeneous materials: the anti-parallel shift ordering of the A-site cations or the existence of chemical cation order in the B-position.