  Measuring the degree of localization of quantum states in phase space is
essential for the description of the dynamics and equilibration of quantum
systems, but this is a topic far from understood. There is no unique way to
measure localization, and individual measures can reflect different aspects of
the same quantum state. Here, we present a general scheme to define
localization in measure spaces, which is based on what we call R\'enyi
occupations, and from which any measure of localization can be derived. We
apply this scheme to the four-dimensional unbounded phase space of the
interacting spin-boson Dicke model. In particular, we make a detailed
comparison of two localization measures based on the Husimi function in the
regime where the model is chaotic, namely one that projects the Husimi function
over the finite phase space of the spin, and another one that uses the Husimi
function defined over classical energy shells. We elucidate the origin of their
differences, showing that in unbounded spaces, to define maximal delocalization
requires a bounded reference subspace, with different selections leading to
contextual answers.
