Phases are groupings of components that share characteristics causing them to behave in a similar way through some fundamental unit operations.
Correctly grouping components in the appropriate phase greatly simplifies building powerful Metsim models.
The most obvious classification of shared characteristic is "tendency to hold shape, density, and initial position" - which results in the classification of phases such as solid, liquid, and gas.
Then there are other classifications of significant usefulness such as organic and inorganic, and special termininology for specific industries and technologies (still with usefulness in separations) such as matte, molten salts, and slag.
It can be seen that the definition of a phase can change with technology development, and processes such as solvent extraction create the need (or usefulness) of new or different groupings of components into phases for specific modelling requirements.
Metsim provides the flexibility and simplicity of using the "out of the box" component characteritics, thermodynamic data, and phase groupings with the appropriate separation behaviour in the rich set of unit operations models, but there is alsoe the flexibility to use the phase groupings to suit your specific requirements and to model new processes which require unique groupings for separations using new characteristics.
In all cases where there are phase changes the thermodynamic data must be correct for calculation of the heat and mass balance.