  Within the theory of statistical ensemble, the so-called $\mu PT$ ensemble
describes equilibrium systems that exchange energy, particles, and volume with
the surrounding. General, model-independent features of volume and particle
number statistics are derived. Non-analytic points of the partition function
are discussed in connection with divergent fluctuations and ensemble
equivalence. Quantum and classical ideal gases, and a model of Bose gas with
mean-field interactions are discussed as examples of the above considerations.
