  One of the most basic notions in physics is the partitioning of a system into
subsystems, and the study of correlations among its parts. In this work, we
explore these notions in the context of quantum reference frame (QRF)
covariance, in which this partitioning is subject to a symmetry constraint. We
demonstrate that different reference frame perspectives induce different sets
of subsystem observable algebras, which leads to a gauge-invariant,
frame-dependent notion of subsystems and entanglement. We further demonstrate
that subalgebras which commute before imposing the symmetry constraint can
translate into non-commuting algebras in a given QRF perspective after symmetry
imposition. Such a QRF perspective does not inherit the distinction between
subsystems in terms of the corresponding tensor factorizability of the
kinematical Hilbert space and observable algebra. Since the condition for this
to occur is contingent on the choice of QRF, the notion of subsystem locality
is frame-dependent.
