  Investigation of simple far-from-equilibrium systems exhibiting phase
separation leads to the conclusion that phase coexistence is not well defined
in this context. This is because the properties of the coexisting
nonequilibrium systems depend on how they are placed in contact, as verified in
the driven lattice gas with attractive interactions, and in the two-temperature
lattice gas, under (a) weak global exchange between uniform systems, and (b)
phase-separated (nonuniform) systems. Thus, far from equilibrium, the notions
of universality of phase coexistence (i.e., independence of how systems
exchange particles and/or energy), and of phases with intrinsic properties
(independent of their environment) are lost.
