  We describe the physical relativity of light and matter quantum subsystems,
their correlations, and energy exchanges. We examine the most commonly adopted
definitions of atoms and photons, noting the significant difference in their
localisation properties when expressed in terms of primitive manifestly
gauge-invariant and local fields. As a result, different behaviours for
entanglement generation and energy exchange occur for different definitions. We
explore such differences in detail using toy models of a single photonic mode
interacting with one and two dipoles.
