The fugacity capacity constant (Z) is used to help describe the concentration of a chemical in a system (usually in mol/mPa). Hemond and Hechner-Levy (2000) describe how to utilize the fugacity capacity to calculate the concentration of a chemical in a system. Depending on the chemical, fugacity capacity varies. The concentration in media m equals the fugacity capacity in media m multiplied by the fugacity of the chemical.
For a chemical system at equilibrium, the fugacity of the chemical will be the same in each media/phase/compartment. Therefore equilibrium is sometimes called "equifugacity" in the context of these calculations.
where Z is a proportional constant, termed fugacity capacity. This equation does not necessarily imply that C and f are always linearly related. Non-linearity can be accommodated by allowing Z to vary as a function of C or f.
For a better understanding of the fugacity capacity concept, heat capacity may provide a precedent for introducing Z as a capacity of a phase to absorb particular quantity of chemical. However, phases with high fugacity capacity do not necessarily retain high fugacity.  
In calculations of fugacity capacity key factors would be (a) the nature of the solute (chemical), (b) the nature of the medium or compartment, (c) temperature.