  Gauge freedom in quantum electrodynamics (QED) outside of textbook regimes is
reviewed. It is emphasized that QED subsystems are defined relative to a choice
of gauge. Each definition uses different gauge-invariant observables. This
relativity is eliminated only if a sufficient number of Markovian and
weak-coupling approximations are employed. All physical predictions are gauge
invariant, including subsystem properties such as photon number and
entanglement. However, subsystem properties naturally differ for different
physical subsystems. Gauge ambiguities arise not because it is unclear how to
obtain gauge-invariant predictions, but because it is not always clear which
physical observables are the most operationally relevant. The gauge invariance
of a prediction is necessary but not sufficient to ensure its operational
relevance. It is shown that, in controlling which gauge invariant observables
are used to define a material system, the choice of gauge affects the balance
between the material system's localization and its electromagnetic dressing.
Various implications of subsystem gauge relativity for deriving effective
models, for describing time-dependent interactions, for photodetection theory,
and for describing matter within a cavity are reviewed.
