  Generalized noncontextuality is a well-studied notion of classicality that is
applicable to a single system, as opposed to Bell locality. It relies on
representing operationally indistinguishable procedures identically in an
ontological model. However, operational indistinguishability depends on the set
of operations that one may use to distinguish two procedures: we refer to this
set as the reference of indistinguishability. Thus, whether or not a given
experiment is noncontextual depends on the choice of reference. The choices of
references appearing in the literature are seldom discussed, but typically
relate to a notion of system underlying the experiment. This shift in
perspective then begs the question: how should one define the extent of the
system underlying an experiment? Our paper primarily aims at exposing this
question rather than providing a definitive answer to it. We start by
formulating a notion of relative noncontextuality for prepare-and-measure
scenarios, which is simply noncontextuality with respect to an explicit
reference of indistinguishability. We investigate how verdicts of relative
noncontextuality depend on this choice of reference, and in the process
introduce the concept of the noncontextuality graph of a prepare-and-measure
scenario. We then discuss several proposals that one may appeal to in order to
fix the reference to a specific choice, and relate these proposals to different
conceptions of what a system really is. With this discussion, we advocate that
whether or not an experiment is noncontextual is not as absolute as often
perceived.
