  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 topic is far from being 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, 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 that uses the Husimi function defined over
classical energy shells. We elucidate the origin of their differences, showing
that in unbounded spaces the definition of maximal delocalization requires a
bounded reference subspace, with different selections leading to contextual
answers.
