  The problem of the possible creation of mixed hadron-quark-gluon matter, that
can arise at nuclear or heavy-ion collisions, is addressed. It is shown that
there can exist several different kinds of such a mixed matter. The main types
of this matter can be classified onto macroscopic mixture, mesoscopic mixture,
and microscopic mixture. Different types of these mixtures require principally
different descriptions. Before comparing theoretical results with experiments,
one has to analyze thermodynamic stability of all these mixed states,
classifying them onto unstable, metastable, and stable. Only the most stable
mixed state should be compared with experiment. Mixed states also need to be
checked with regard to stratification instability. In addition to the static
stratification instability, there can happen dynamic instability occurring in a
mixture of components moving with respect to each other. This effect, called
counterflow instability, has also to be taken into account, since it can lead
to the stratification of mixed matter.
